[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, 1 deletions

diff --git a/clang/unittests/Frontend/CompilerInvocationTest.cpp b/clang/unittests/Frontend/CompilerInvocationTest.cpp
index 94ab9fe..75390aa 100644
--- a/clang/unittests/Frontend/CompilerInvocationTest.cpp
+++ b/clang/unittests/Frontend/CompilerInvocationTest.cpp
@@ -29,6 +29,7 @@ using ::testing::StartsWith;
 namespace {
 class CommandLineTest : public ::testing::Test {
 public:
+  DiagnosticOptions DiagOpts;
   IntrusiveRefCntPtr<DiagnosticsEngine> Diags;
   SmallVector<const char *, 32> GeneratedArgs;
   BumpPtrAllocator Alloc;
@@ -41,7 +42,7 @@ public:
 
   CommandLineTest()
       : Diags(CompilerInstance::createDiagnostics(
-            *llvm::vfs::getRealFileSystem(), new DiagnosticOptions(),
+            *llvm::vfs::getRealFileSystem(), DiagOpts,
             new TextDiagnosticBuffer())),
         StringPool(Alloc) {}
 };