Revert "[OpenMP][NFCI] Move OpenMP clause information to `lib/Frontend/OpenMP`"

This reverts commit c18d55998b3352e6ec92ccb8a3240a16a57c61e6.

Bots have reported uses that need changing, e.g.,
  clang-tools-extra/clang-tidy/openmp/UseDefaultNoneCheck.cp
as reported by
  http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/46591
diff --git a/clang/lib/AST/AttrImpl.cpp b/clang/lib/AST/AttrImpl.cpp
index 7d3a290..a5ff68c 100644
--- a/clang/lib/AST/AttrImpl.cpp
+++ b/clang/lib/AST/AttrImpl.cpp
@@ -108,8 +108,7 @@
   for (auto *E : linears()) {
     OS << " linear(";
     if (*MI != OMPC_LINEAR_unknown)
-      OS << getOpenMPSimpleClauseTypeName(llvm::omp::Clause::OMPC_linear, *MI)
-         << "(";
+      OS << getOpenMPSimpleClauseTypeName(OMPC_linear, *MI) << "(";
     E->printPretty(OS, nullptr, Policy);
     if (*MI != OMPC_LINEAR_unknown)
       OS << ")";