This avoids breaking clang-tidy/infrastructure/validate-check-names.cpp
if 'not' is evaluated as a lit internal tool (making TestRunner
invoke 'grep' directly in that test, instead of invoking 'not', which
then invokes 'grep').
The quoting of arguments is still brittle if the executable is an
MSYS based tool though, as MSYS based tools incorrectly unescape
backslashes in quoted arguments (contrary to regular win32 argument
parsing rules), see D99406 and
https://github.com/msys2/msys2-runtime/issues/36 for more examples
of the issues.