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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

To put it another way, what do we want to prevent the user from accomplishing if he obtains a non-const reference to ASTContext from a const ASTUnit?

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