[Macros] Don't allow local names to shadow macro names.

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -3667,10 +3667,11 @@ namespace {
     ) {
       auto result = TypeChecker::lookupUnqualified(
           CurDC, DeclNameRef(macroName), loc,
-          defaultUnqualifiedLookupOptions);
+          (defaultUnqualifiedLookupOptions |
+           NameLookupFlags::IncludeOuterResults));
 
       SmallVector<OverloadChoice, 1> choices;
-      for (const auto &found : result) {
+      for (const auto &found : result.allResults()) {
         if (auto macro = dyn_cast<MacroDecl>(found.getValueDecl())) {
           OverloadChoice choice = OverloadChoice(Type(), macro, functionRefKind);
           choices.push_back(choice);

test/Macros/macros_diagnostics.swift

@@ -1,7 +1,7 @@
 // RUN: %target-typecheck-verify-swift -enable-experimental-feature Macros -module-name MacrosTest
 // REQUIRES: OS=macosx
 
-macro stringify<T>(_ value: T) -> (T, String) = BuiltinMacros.StringifyMacro
+macro stringify<T>(_ value: T) -> (T, String) = _SwiftSyntaxMacros.StringifyMacro
 
 protocol P { }
 
@@ -35,3 +35,6 @@ func test(a: Int, b: Int) {
   let s = #stringify<Int, Int>(a + b) // expected-error{{type of expression is ambiguous without more context}}
 }
 
+func shadow(a: Int, b: Int, stringify: Int) {
+  _ = #stringify(a + b)
+}