[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, 6 insertions, 7 deletions

diff --git a/clang/lib/Frontend/ASTMerge.cpp b/clang/lib/Frontend/ASTMerge.cpp
index b6b0644..a4ce883 100644
--- a/clang/lib/Frontend/ASTMerge.cpp
+++ b/clang/lib/Frontend/ASTMerge.cpp
@@ -41,14 +41,13 @@ void ASTMergeAction::ExecuteAction() {
   auto SharedState = std::make_shared<ASTImporterSharedState>(
       *CI.getASTContext().getTranslationUnitDecl());
   for (unsigned I = 0, N = ASTFiles.size(); I != N; ++I) {
-    IntrusiveRefCntPtr<DiagnosticsEngine>
-        Diags(new DiagnosticsEngine(DiagIDs, &CI.getDiagnosticOpts(),
-                                    new ForwardingDiagnosticConsumer(
-                                          *CI.getDiagnostics().getClient()),
-                                    /*ShouldOwnClient=*/true));
+    IntrusiveRefCntPtr<DiagnosticsEngine> Diags(new DiagnosticsEngine(
+        DiagIDs, CI.getDiagnosticOpts(),
+        new ForwardingDiagnosticConsumer(*CI.getDiagnostics().getClient()),
+        /*ShouldOwnClient=*/true));
     std::unique_ptr<ASTUnit> Unit = ASTUnit::LoadFromASTFile(
-        ASTFiles[I], CI.getPCHContainerReader(), ASTUnit::LoadEverything, Diags,
-        CI.getFileSystemOpts(), CI.getHeaderSearchOpts());
+        ASTFiles[I], CI.getPCHContainerReader(), ASTUnit::LoadEverything,
+        nullptr, Diags, CI.getFileSystemOpts(), CI.getHeaderSearchOpts());
 
     if (!Unit)
       continue;