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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

This is weakening const correctness, which is the opposite direction of where we should be going IMO.

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