[llvm-branch-commits] [llvm] Use lit internal shell by default (PR #157237)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/157237

This should result in a 10-15% test time improvement and also richer
test failure outputn on Linux where it is not yet the default. We can do
this now that all of the tests have been ported over to work with lit's
internal shell.

Fixes #102697.