[llvm-branch-commits] [Flang] Enable lit's internal shell by default (PR #156116)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/156116

Lit's internal shell is faster as it avoids the overhead of needing to
fork bash for every run line, and also provides better debug information
on failures.

Now that all of flang's tests work correctly with lit's internal shell,
we can enable it by default to take advantage of the benefits.