[CIR][NFC] Rename variables to avoid gcc compilation error (#1982)

The repo no longer builds with gcc:

```
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:113:21: error: declaration of ‘clang::CompilerInstance& cir::CIRGenConsumer::CompilerInstance’ changes meaning of ‘CompilerInstance’ [-fpermissive]
  113 |   CompilerInstance &CompilerInstance;
      |                     ^~~~~~~~~~~~~~~~
In file included from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/FrontendAction.h:24,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CodeGen/CodeGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CIR/FrontendAction/CIRGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:9:
/usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/CompilerInstance.h:81:7: note: ‘CompilerInstance’ declared here as ‘class clang::CompilerInstance’
   81 | class CompilerInstance : public ModuleLoader {
      |       ^~~~~~~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:114:22: error: declaration of ‘clang::DiagnosticsEngine& cir::CIRGenConsumer::DiagnosticsEngine’ changes meaning of ‘DiagnosticsEngine’ [-fpermissive]
  114 |   DiagnosticsEngine &DiagnosticsEngine;
      |                      ^~~~~~~~~~~~~~~~~
In file included from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/AST/NestedNameSpecifier.h:18,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/AST/Type.h:21,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/AST/CanonicalType.h:17,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/AST/ASTContext.h:18,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/ASTUnit.h:17,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/FrontendAction.h:23,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CodeGen/CodeGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CIR/FrontendAction/CIRGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:9:
/usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Basic/Diagnostic.h:230:7: note: ‘DiagnosticsEngine’ declared here as ‘class clang::DiagnosticsEngine’
  230 | class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
      |       ^~~~~~~~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:115:47: warning: ‘maybe_unused’ attribute ignored [-Wattributes]
  115 |   [[maybe_unused]] const HeaderSearchOptions &HeaderSearchOptions;
      |                                               ^~~~~~~~~~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:115:47: error: declaration of ‘const clang::HeaderSearchOptions& cir::CIRGenConsumer::HeaderSearchOptions’ changes meaning of ‘HeaderSearchOptions’ [-fpermissive]
In file included from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/ASTUnit.h:25,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/FrontendAction.h:23,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CodeGen/CodeGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CIR/FrontendAction/CIRGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:9:
/usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Lex/HeaderSearchOptions.h:64:7: note: ‘HeaderSearchOptions’ declared here as ‘class clang::HeaderSearchOptions’
   64 | class HeaderSearchOptions {
      |       ^~~~~~~~~~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:116:19: error: declaration of ‘clang::CodeGenOptions& cir::CIRGenConsumer::CodeGenOptions’ changes meaning of ‘CodeGenOptions’ [-fpermissive]
  116 |   CodeGenOptions &CodeGenOptions;
      |                   ^~~~~~~~~~~~~~
In file included from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Basic/TargetInfo.h:21,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/CompilerInstance.h:15,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/FrontendAction.h:24,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CodeGen/CodeGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CIR/FrontendAction/CIRGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:9:
/usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Basic/CodeGenOptions.h:88:7: note: ‘CodeGenOptions’ declared here as ‘class clang::CodeGenOptions’
   88 | class CodeGenOptions : public CodeGenOptionsBase {
      |       ^~~~~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:117:41: warning: ‘maybe_unused’ attribute ignored [-Wattributes]
  117 |   [[maybe_unused]] const TargetOptions &TargetOptions;
      |                                         ^~~~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:117:41: error: declaration of ‘const clang::TargetOptions& cir::CIRGenConsumer::TargetOptions’ changes meaning of ‘TargetOptions’ [-fpermissive]
In file included from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/ASTUnit.h:24,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Frontend/FrontendAction.h:23,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CodeGen/CodeGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/include/clang/CIR/FrontendAction/CIRGenAction.h:12,
                 from /usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:9:
/usr/WS2/koparasy/LLVM/clangir/clang/include/clang/Basic/TargetOptions.h:26:7: note: ‘TargetOptions’ declared here as ‘class clang::TargetOptions’
   26 | class TargetOptions {
      |       ^~~~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:118:39: warning: ‘maybe_unused’ attribute ignored [-Wattributes]
  118 |   [[maybe_unused]] const LangOptions &LangOptions;
      |                                       ^~~~~~~~~~~
/usr/WS2/koparasy/LLVM/clangir/clang/lib/CIR/FrontendAction/CIRGenAction.cpp:118:39: error: declaration of ‘const clang::LangOptions& cir::CIRGenConsumer::LangOptions’ changes meaning of ‘LangOptions’ [-fpermissive]

```

This fixes these variable naming issues.

Author: koparasy