[clang] Hide the DiagnosticOptions pointer from CompilerInvocation (llvm#106274)

This PR hides the reference-counter pointer that holds
`DiagnosticOptions` from the public API of `CompilerInvocation`. This
gives `CompilerInvocation` an exclusive control over the lifetime of
this member, which will eventually be leveraged to implement a
copy-on-write behavior.

The only client that currently accesses that pointer is
`clangd::buildPreamble()` which takes care to reset it so that it's not
reset concurrently. This code is made redundant by making the reference
count of `DiagnosticOptions` atomic.

Author: jansvoboda11

clang-tools-extra/clangd/Preamble.cpp

@@ -673,7 +673,6 @@ buildPreamble(PathRef FileName, CompilerInvocation CI,
   // Reset references to ref-counted-ptrs before executing the callbacks, to
   // prevent resetting them concurrently.
   PreambleDiagsEngine.reset();
-  CI.DiagnosticOpts.reset();
 
   // When building the AST for the main file, we do want the function
   // bodies.

clang/include/clang/Basic/DiagnosticOptions.h

@@ -67,7 +67,8 @@ inline DiagnosticLevelMask operator&(DiagnosticLevelMask LHS,
 raw_ostream& operator<<(raw_ostream& Out, DiagnosticLevelMask M);
 
 /// Options for controlling the compiler diagnostics engine.
-class DiagnosticOptions : public RefCountedBase<DiagnosticOptions>{
+class DiagnosticOptions
+    : public llvm::ThreadSafeRefCountedBase<DiagnosticOptions> {
   friend bool ParseDiagnosticArgs(DiagnosticOptions &, llvm::opt::ArgList &,
                                   clang::DiagnosticsEngine *, bool);
 

clang/include/clang/Frontend/CompilerInvocation.h

@@ -269,7 +269,6 @@ class CompilerInvocation : public CompilerInvocationBase {
   /// @{
   using CompilerInvocationBase::LangOpts;
   using CompilerInvocationBase::TargetOpts;
-  using CompilerInvocationBase::DiagnosticOpts;
   std::shared_ptr<HeaderSearchOptions> getHeaderSearchOptsPtr() {
     return HSOpts;
   }