Revert "[lit] Handle plain negations directly in the internal shell"

This reverts commit d09adfd3.

That commit caused failures in
clang-tidy/infrastructure/validate-check-names.cpp on windows
buildbots.

That change exposed a surprising issue, not directly related to
this change in itself, but in how TestRunner quotes command line
arguments that later are going to be interpreted by a msys based
tool (like grep.exe, when provided by Git for Windows). This
worked accidentally before, when grep was invoked via not.exe
which took a more conservative approach to windows argument quoting.