[lit] Fix internal env calling other internal commands

Without this patch, when using lit's internal shell, if `env` on a lit
RUN line calls `cd`, `mkdir`, or any of the other in-process shell
builtins that lit implements, lit accidentally searches for the latter
as an external executable.

This patch puts such builtins in a map so that boilerplate for them
need be implemented only once.  This patch moves that handling after
processing of `env` so that `env` calling such a builtin can be
detected.  Finally, because such calls appear to be useless, this
patch takes the safe approach of diagnosing them rather than
supporting them.

Reviewed By: probinson, mgorny, rnk

Differential Revision: https://reviews.llvm.org/D66506