[lit] Handle plain negations directly in the internal shell

Keep running "not --crash" via the external "not" executable, but
for plain negations, and for cases that use the shell "!" operator,
just skip that argument and invert the return code.

The libcxx tests only use the shell operator "!" for negations,
never the "not" executable, because libcxx tests can be run without
having a fully built llvm tree available providing the "not"
executable.

This allows using the internal shell for libcxx tests.

Differential Revision: https://reviews.llvm.org/D98859