[clang] [clang] Clear ASTContext::TUDecl in 'cleanup' for easier debugging (PR #191697)

[Balázs Benics via cfe-commits]

steakhal wrote:

> > I'm glad to see alignment here. Thank you. I didn't expect such a great interest. In this case, I'd raise another point (but without expanding the current PR):
> > In the `CodeGenAction.cpp` we have this code:
> > [...]
> > And this raises me a question, why is `C.getAllocator().Reset();` not done part of `cleanup` already? IMO it's very suspicious.
> > What I suspected was that it is used from `ASTContext::PrintStats` thus reseting it would have an effect on debuggability. However, this `ASTContext::PrintStats` is called from `clang::ParseAST` after `Consumer->HandleTranslationUnit(S.getASTContext());` is called. This means that even today, this allocator is conditionally reset. So I really have no idea why it's not in `cleanup`; but I think I'll leave it for the experts to judge.
> 
> Resetting the allocator is a pretty expensive operation, so we choose not to (and prefer to just 'leak' or continue allocation) unless we opt-in. This is one of those things that has significant compile-time implications.

Ah I see. Well, it might deserve a comment then.

https://github.com/llvm/llvm-project/pull/191697