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

This swaps out the OpenMPDefaultClauseKind enum with a
llvm::omp::DefaultKind enum which is stored in OMPConstants.h.

This should not change any functionality.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D74513
diff --git a/clang/lib/AST/OpenMPClause.cpp b/clang/lib/AST/OpenMPClause.cpp
index 1cd1c82..c5c9bc7 100644
--- a/clang/lib/AST/OpenMPClause.cpp
+++ b/clang/lib/AST/OpenMPClause.cpp
@@ -1244,7 +1244,8 @@
 
 void OMPClausePrinter::VisitOMPDefaultClause(OMPDefaultClause *Node) {
   OS << "default("
-     << getOpenMPSimpleClauseTypeName(OMPC_default, Node->getDefaultKind())
+     << getOpenMPSimpleClauseTypeName(OMPC_default,
+                                      unsigned(Node->getDefaultKind()))
      << ")";
 }