[llvm-branch-commits] [Clang] Enable lit internal shell by default (PR #158465)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/158465

Now that all of the clang tests have been verified (and adjusted when
necessary) to work with lit's internal shell, we can enable it by
default, which this patch does.

This should make check-clang 10-15% faster in addition to providing
richer feedback on test failures.