clang-cl: Warn about /U flags that look like filenames (PR31662)

Both on Mac and Windows, it's common to have a 'Users' directory in the
root of the filesystem, so one might specify a filename as
'/Users/me/myfile.c'. clang-cl (as well as MSVC's cl.exe) will interpret
that as invoking the '/U' option, which is probably not what the user
wanted. Add a warning about this.

Differential Revision: https://reviews.llvm.org/D29198

llvm-svn: 293305