[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/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index fbd59b9..fa5b43d 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -11887,8 +11887,7 @@
 }
 
 void OMPClauseReader::VisitOMPDefaultClause(OMPDefaultClause *C) {
-  C->setDefaultKind(
-       static_cast<OpenMPDefaultClauseKind>(Record.readInt()));
+  C->setDefaultKind(static_cast<llvm::omp::DefaultKind>(Record.readInt()));
   C->setLParenLoc(Record.readSourceLocation());
   C->setDefaultKindKwLoc(Record.readSourceLocation());
 }