lit.util.which('link') picks up the wrong link.exe in git bash, leading
to this error:
# command stderr:
/usr/bin/link: extra operand '/LIBPATH:C:\\Progra....'
Try '/usr/bin/link --help' for more information.
Instead, assume that link.exe is next to cl.exe.
Differential Revision: https://reviews.llvm.org/D109832