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

[Boaz Brickner via cfe-commits]

bricknerb 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()`.

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