[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:
```c++
  if (CodeGenOpts.ClearASTBeforeBackend) {
    LLVM_DEBUG(llvm::dbgs() << "Clearing AST...\n");
    // Access to the AST is no longer available after this.
    // Other things that the ASTContext manages are still available, e.g.
    // the SourceManager. It'd be nice if we could separate out all the
    // things in ASTContext used after this point and null out the
    // ASTContext, but too many various parts of the ASTContext are still
    // used in various parts.
    C.cleanup();
    C.getAllocator().Reset(); // <--- here
  }
```
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.

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