[clang] Change `ASTUnit::getASTContext() const` to return a non-const `ASTContext` (PR #130096)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Thanks for the feedback!
> 
> Is there a way to call `ASTContext::createMangleContext()` when I have a `const ASTUnit`? I've tried to make `ASTContext::createMangleContext()` const but this failed as it seems to actually call methods that potentially modify the context.
> 
> However, when I have a `const Decl`, I can just call `getAstContext().createMangleContext()`.

There's two answers. In a world where we have const correctness, you shouldn't be able to (this is a misfeature on `Decl` that it has a `const` overload that returns a `const ASTContext &`). However, Clang inherited a lack of const correctness from the mists of time, so we have these sort of misfeatures still lurking (and will for quite some time). So for now, the recommendation is `const_cast` so it's clear where we're breaking const correctness.

https://github.com/llvm/llvm-project/pull/130096