[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
diff --git a/clang/lib/AST/OpenMPClause.cpp b/clang/lib/AST/OpenMPClause.cpp
index 30d55ef..5ef8264 100644
--- a/clang/lib/AST/OpenMPClause.cpp
+++ b/clang/lib/AST/OpenMPClause.cpp
@@ -1240,7 +1240,8 @@
 
 void OMPClausePrinter::VisitOMPProcBindClause(OMPProcBindClause *Node) {
   OS << "proc_bind("
-     << getOpenMPSimpleClauseTypeName(OMPC_proc_bind, Node->getProcBindKind())
+     << getOpenMPSimpleClauseTypeName(OMPC_proc_bind,
+                                      unsigned(Node->getProcBindKind()))
      << ")";
 }