[Macros] Be deliberate about walking macro arguments vs. expansions

Provide ASTWalker with a customization point to specify whether to
check macro arguments (which are type checked but never emitted), the
macro expansion (which is the result of applying the macro and is
actually emitted into the source), or both. Provide answers for the
~115 different ASTWalker visitors throughout the code base.

Fixes rdar://104042945, which concerns checking of effects in
macro arguments---which we shouldn't do.

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -969,8 +969,9 @@ public:
              llvm::MutableArrayRef<TextEntity*> FuncEnts)
     : SM(SM), BufferID(BufferID), FuncEnts(FuncEnts) {}
 
-  bool shouldWalkMacroExpansions() override {
-    return false;
+  /// Only walk the arguments of a macro, to represent the source as written.
+  MacroWalking getMacroWalkingBehavior() const override {
+    return MacroWalking::ArgumentsAndExpansion;
   }
 
   PreWalkAction walkToDeclPre(Decl *D) override {