[OpenMP][NFCI] Use the libFrontend ProcBindKind in Clang

This removes the OpenMPProcBindClauseKind enum in favor of
llvm::omp::ProcBindKind which lives in OpenMPConstants.h and was
introduced in D70109.

No change in behavior is expected.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D70289