Add missing #include for ObjCMethodDecl.

DeclBase.h only contains a forward declaration of ObjCMethodDecl, and
when building clang/Sema/Attr.h with header modules this causes a build
failure because `llvm::isa<ObjCMethodDecl>` requires the full type.

Author: slackito

clang/include/clang/Sema/Attr.h

@@ -14,7 +14,7 @@
 #define LLVM_CLANG_SEMA_ATTR_H
 
 #include "clang/AST/Decl.h"
-#include "clang/AST/DeclBase.h"
+#include "clang/AST/DeclObjC.h"
 #include "llvm/Support/Casting.h"
 
 namespace clang {