[lit] Fix `not` calling internal commands

[lit] Fix `not` calling internal commands

Without this patch, when using lit's internal shell, if `not` on a lit
RUN line calls `env`, `diff`, or any of the other in-process shell
builtins that lit implements, lit accidentally searches for the latter
as an external executable.  What's worse is that works fine when a
developer is testing on a platform where those executables are
available and behave as expected, but it then breaks on other
platforms.

`not` seems useful for some builtins, such as `diff`, so this patch
supports such uses.  `not --crash` does not seem useful for builtins,
so this patch diagnoses such uses.  In all cases, this patch ensures
shell builtins are found behind any sequence of `env` and `not`
commands.

`not` calling `env` calling an external command appears useful when
the `env` and external command are part of a lit substitution, as in
D65156.  This patch supports that by looking through any sequence of
`env` and `not` commands, building the environment from the `env`s,
and storing the `not`s.  The `not`s are then added back to the command
line without the `env`s to execute externally.  This avoids the need
to replicate the `not` implementation, in particular the `--crash`
option, in lit.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D66531