cpp: support vendor'ed clang
cpp: support vendor'ed clang

It is possible to build clang with an embedded vendor tag.  In such a
scenario the version string returned contains the vendor.  Adjust the
version checking logic to be more lenient against such a configuration.

Closes #15269.

PiperOrigin-RevId: 448921259

bazelbuild/bazel,bazelbuild/bazel,katre/bazel,katre/bazel,cushon/bazel,cushon/bazel,katre/bazel,bazelbuild/bazel,katre/bazel,cushon/bazel,katre/bazel,cushon/bazel,cushon/bazel,cushon/bazel,bazelbuild/bazel,bazelbuild/bazel,katre/bazel,bazelbuild/bazel