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.
9e306ad460