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

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Just throwing in my $0.02 and saying I agree w/ Aaron and Vlad here, this is not the right direction. Having a `const` method return a non-const that we will then mutate is just not clean code.

Having to use `const_cast` documents we are violating expectations and marks this as something we need to clean up in the future.

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