[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/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 258db0d..a9f433b 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -11771,8 +11771,7 @@
 }
 
 void OMPClauseReader::VisitOMPProcBindClause(OMPProcBindClause *C) {
-  C->setProcBindKind(
-       static_cast<OpenMPProcBindClauseKind>(Record.readInt()));
+  C->setProcBindKind(static_cast<llvm::omp::ProcBindKind>(Record.readInt()));
   C->setLParenLoc(Record.readSourceLocation());
   C->setProcBindKindKwLoc(Record.readSourceLocation());
 }