Sink more Attr.h inline methods, NFC

This has very little impact on build time, but is a mechanical pre-req
to removing the OpenMPClause.h include, which matters. Most of these
pretty print methods require Expr to be complete.
diff --git a/clang/lib/AST/OpenMPClause.cpp b/clang/lib/AST/OpenMPClause.cpp
index 00d4342..e68a5d6 100644
--- a/clang/lib/AST/OpenMPClause.cpp
+++ b/clang/lib/AST/OpenMPClause.cpp
@@ -12,6 +12,7 @@
 
 #include "clang/AST/OpenMPClause.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/Attr.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclOpenMP.h"
 #include "clang/Basic/LLVM.h"