FindOpenMP: Prefer Clang's own libomp when targeting MSVC

Clang targeting the MSVC ABI has no implicit link directories, so the
search fell through to `LIB` and picked up MSVC's `libomp.lib`, which
needs the non-redistributable `libomp140` DLL, or found nothing outside
a VS environment.  The library next to the compiler now comes first,
same as clang's driver, with the old search as a fallback when it
doesn't link, e.g. for a cross target.

The docs now say `OpenMP_RUNTIME_MSVC` is only for `cl`, and clang-cl
no longer tries the GNU-style flags it ignores.

Fixes: #28133