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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

There is no const_cast, or casting away const in any form on this patch, so how does it weaken const correctness?

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