[llvm-branch-commits] [MC] Invoke run line in stdin.s using bash (PR #157232)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/157232

This test needs to use subshells so that the file descriptor offset gets
moved before llvm-mc starts writing output. I cannot get this to trigger
the assertion (after reverting the code fixes that fixed this behavior)
without using subshells. Various other forms of redirection leave the
offset at zero. All systems used for testing LLVM will have a bash
binary given that's what is used by llvm-lit currently.

This does not address the problem of test coverage on more esoteric
platforms like Fuchsia, but running this one test on more esoteric
platforms probably does not matter much.