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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll requested changes to this pull request.

I agree with Aaron that this patch doesn't move us towards the direction we generally want. I'd like to see more justification than 2 lines in the description.

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