[clang] Remove intrusive reference count from `DiagnosticOptions` (#139584)

The `DiagnosticOptions` class is currently intrusively
reference-counted, which makes reasoning about its lifetime very
difficult in some cases. For example, `CompilerInvocation` owns the
`DiagnosticOptions` instance (wrapped in `llvm::IntrusiveRefCntPtr`) and
only exposes an accessor returning `DiagnosticOptions &`. One would
think this gives `CompilerInvocation` exclusive ownership of the object,
but that's not the case:

```c++
void shareOwnership(CompilerInvocation &CI) {
  llvm::IntrusiveRefCntPtr<DiagnosticOptions> CoOwner = &CI.getDiagnosticOptions();
  // ...
}
```

This is a perfectly valid pattern that is being actually used in the
codebase.

I would like to ensure the ownership of `DiagnosticOptions` by
`CompilerInvocation` is guaranteed to be exclusive. This can be
leveraged for a copy-on-write optimization later on. This PR changes
usages of `DiagnosticOptions` across `clang`, `clang-tools-extra` and
`lldb` to not be intrusively reference-counted.

1 files changed, 2 insertions, 2 deletions

diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
index 4028bbf..09e5c65 100644
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -2391,11 +2391,11 @@ SourceManagerForFile::SourceManagerForFile(StringRef FileName,
   // in `Environment` so that `FileMgr` can out-live this function scope.
   FileMgr =
       std::make_unique<FileManager>(FileSystemOptions(), InMemoryFileSystem);
+  DiagOpts = std::make_unique<DiagnosticOptions>();
   // This is passed to `SM` as reference, so the pointer has to be referenced
   // by `Environment` due to the same reason above.
   Diagnostics = std::make_unique<DiagnosticsEngine>(
-      IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs),
-      new DiagnosticOptions);
+      IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs), *DiagOpts);
   SourceMgr = std::make_unique<SourceManager>(*Diagnostics, *FileMgr);
   FileEntryRef FE = llvm::cantFail(FileMgr->getFileRef(FileName));
   FileID ID =