[AST] Allow ignoring macro expansions

Various requests expect to be walking over the current source file.
While we could add checks to all these to skip decls outside of the
current buffer, it's a little nicer to handle this during the walk
instead.

Allow ignoring nodes that are from macro expansions and add that flag to
the various walks that expect it.

Also add a new `getOriginalAttrs` that filters out attributes in
generated source.

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -969,6 +969,10 @@ public:
              llvm::MutableArrayRef<TextEntity*> FuncEnts)
     : SM(SM), BufferID(BufferID), FuncEnts(FuncEnts) {}
 
+  bool shouldWalkMacroExpansions() override {
+    return false;
+  }
+
   PreWalkAction walkToDeclPre(Decl *D) override {
     if (D->isImplicit())
       return Action::SkipChildren(); // Skip body.