[clang][Tooling] Use Windows command lines on all Windows, except Cygwin

Previously it only used Windows command lines for MSVC triples, but this
was causing issues for windows-gnu.  In fact, everything 'native' Windows
(ie, not Cygwin) should use Windows command line parsing.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D111195