[clang] [OpenMP] Diagnose use of 'target_clones' in OpenMP variant declarations.

Previously, OpenMP variant declarations for a function declaration that included
the 'cpu_dispatch', 'cpu_specific', or 'target' attributes was diagnosed, but
one with the 'target_clones' attribute was not. Now fixed.

Reviewed By: erichkeane, jdoerfert

Differential Revision: https://reviews.llvm.org/D121963