[clang][lex] Use FileManager to make prebuilt module paths absolute (#165347)

This PR switches from using `llvm::sys::fs::make_absolute()` to
`FileManager::makeAbsolutePath()` so that `FileSystemOptions` (i.e. the
`-working-directory` option) and the `VFS`'s CWD have a say in how the
prebuilt module paths are resolved. This matches how the rest of the
compiler treats input files.

Author: jansvoboda11

clang/lib/Lex/HeaderSearch.cpp

@@ -221,7 +221,7 @@ std::string HeaderSearch::getPrebuiltModuleFileName(StringRef ModuleName,
   // file.
   for (const std::string &Dir : HSOpts.PrebuiltModulePaths) {
     SmallString<256> Result(Dir);
-    llvm::sys::fs::make_absolute(Result);
+    FileMgr.makeAbsolutePath(Result);
     if (ModuleName.contains(':'))
       // The separator of C++20 modules partitions (':') is not good for file
       // systems, here clang and gcc choose '-' by default since it is not a
@@ -246,7 +246,7 @@ std::string HeaderSearch::getPrebuiltImplicitModuleFileName(Module *Module) {
   StringRef ModuleCacheHash = HSOpts.DisableModuleHash ? "" : getModuleHash();
   for (const std::string &Dir : HSOpts.PrebuiltModulePaths) {
     SmallString<256> CachePath(Dir);
-    llvm::sys::fs::make_absolute(CachePath);
+    FileMgr.makeAbsolutePath(CachePath);
     llvm::sys::path::append(CachePath, ModuleCacheHash);
     std::string FileName =
         getCachedModuleFileNameImpl(ModuleName, ModuleMapPath, CachePath);