[llvm] [Lit][NFC] Refactor shell environment functionality and in-process builtins from TestRunner.py into new modules (PR #176761)

[Benjamin Stott via llvm-commits]

BStott6 wrote:

I rebased the branch and when I pushed it seems everyone who is a code owner for areas touched by commits that were added in the meantime was automatically requested for review - this is strange as I don't even have the privilege to request reviews myself. Really sorry to everyone whose time was wasted by this.

https://github.com/llvm/llvm-project/pull/176761