[Name lookup] Narrow shadowing to non-member types.

Narrow the recently-introduced shadowing rule for types so that it only
applies to non-member types. Member types lack a reasonable syntax for
specifying precisely which module to look into, and there are a few use
cases where the type checker will pick a type that would be shadowed by
the new rule.

Fixes a source-compatibility regression introduced by the shadowing rule.

Author: DougGregor

lib/AST/NameLookup.cpp

@@ -377,6 +377,9 @@ static void recordShadowedDecls(ArrayRef<ValueDecl *> decls,
       // type. This is layering a partial fix upon a total hack.
       if (auto asd = dyn_cast<AbstractStorageDecl>(decl))
         signature = asd->getOverloadSignatureType();
+    } else if (decl->getDeclContext()->isTypeContext()) {
+      // Do not apply shadowing rules for member types.
+      continue;
     }
 
     // Record this declaration based on its signature.

test/NameBinding/Inputs/MemberTypesInClasses.swift

@@ -0,0 +1,7 @@
+public class RootClass {
+}
+
+public class SubClass: RootClass {
+  public typealias Member = Int
+}
+

test/NameBinding/member_type_shadowing.swift

@@ -0,0 +1,15 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/MemberTypesInClasses.swift
+// RUN: %target-swift-frontend -typecheck %s -I %t -verify
+
+import MemberTypesInClasses
+
+protocol P {
+  associatedtype Member
+}
+
+extension RootClass: P {
+  typealias Member = SubClass.Member
+}
+
+