[clang][llvm-lit] Enable internal shell for clang test suite

[Harini0924]

Testing in lit is currently inconsistent across different platforms, as many RUN line commands are written in a way that only works on POSIX-compliant shells. Lit has an internal shell implementation to ensure uniform execution of these commands that can be used regardless of platform. We should aim to turn on the internal shell for the clang test suite to eliminate reliance on external shells.

The internal shell has missing capabilities that need to be addressed before doing so. Here is a list of the issues that block us from turning on the internal shell for clang.

• [llvm-lit] Environment variable not found in LLVM subproject with lit internal shell  #102395
  [llvm-lit] lit internal shell failing to parse and execute command substitution syntax #102384
  [llvm-lit] Unhashable TypeError 'GlobItem' with lit’s internal shell #102389
  [llvm-lit] Lit's built-in diff command causing clang test to fail when run with internal shell #106249