clang-cl: Prefer /vctoolsdir, /winsdkdir over LIB for link invocations

/vctoolsdir and /winsdkdir take precedence over the INCLUDE env var,
so they should also take precedence over LIB. It's not quite as neat
since LIB is still read by the linker and the linker just prefers
the -libpath: paths the driver now passes, but as long as all libraries
are present at /vctoolsdir and /winsdkdir, there's no harm in the linker
also looking at LIB later.

This fixes cl-options.c after a5d85cbe on Windows when LIB is set.
Another way to fix the test would be to prefix the clang-cl
line with `env --unset=LIB`, but I think it's better to fix the
flag to work as expected instead of making the test work around
the surprising behavior that LIB being set causes clang-cl to
not pass -libpath: flags to the linker when /vctoolsdir and
/winsdkdir are used.