[lit] Implement non-pipelined ‘mkdir’, ‘diff’ and ‘rm’ commands internally

Summary:
The internal shell already supports 'cd', ‘export’ and ‘echo’ commands.
This patch adds implementation of non-pipelined ‘mkdir’, ‘diff’ and ‘rm’
commands as the internal shell builtins.

Reviewed by: Zachary Turner, Reid Kleckner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39567

llvm-svn: 319528