[Sema] Scale back CodingKeys hack in TypeChecker::lookupUnqualifiedType

I previously added this hack to match the logic in
`TypeChecker::lookupUnqualified`, but it turns out that can introduce
request cycles for cases where `CodingKeys` is used in a generic
requirement for one of `Codable`'s potential value witnesses. Scale
back the hack such that it's only done when we get an initial empty
lookup result, ensuring we maintain source compatibility. Both these
lookup hacks should go away once we properly handle CodingKeys 
synthesis.

rdar://153096639

Author: hamishknight

lib/Sema/TypeCheckNameLookup.cpp

@@ -325,9 +325,6 @@ TypeChecker::lookupUnqualifiedType(DeclContext *dc, DeclNameRef name,
                                    NameLookupOptions options) {
   auto &ctx = dc->getASTContext();
 
-  // HACK: Synthesize CodingKeys if needed.
-  synthesizeCodingKeysIfNeededForUnqualifiedLookup(ctx, dc, name);
-
   auto ulOptions = convertToUnqualifiedLookupOptions(options) |
                    UnqualifiedLookupFlags::TypeLookup;
   {
@@ -336,8 +333,15 @@ TypeChecker::lookupUnqualifiedType(DeclContext *dc, DeclNameRef name,
         name, dc, loc,
         ulOptions - UnqualifiedLookupFlags::AllowProtocolMembers);
 
-    auto lookup =
-        evaluateOrDefault(ctx.evaluator, UnqualifiedLookupRequest{desc}, {});
+    UnqualifiedLookupRequest req(desc);
+    auto lookup = evaluateOrDefault(ctx.evaluator, req, {});
+
+    // HACK: Try synthesize CodingKeys if we got an empty result.
+    if (lookup.allResults().empty() && name.isSimpleName(ctx.Id_CodingKeys)) {
+      synthesizeCodingKeysIfNeededForUnqualifiedLookup(ctx, dc, name);
+      lookup = evaluateOrDefault(ctx.evaluator, req, {});
+    }
+
     if (!lookup.allResults().empty())
       return lookup;
   }

test/Sema/rdar153096639.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift
+
+// Make sure we don't run into a request cycle for the below use of 'CodingKeys'
+// in an `init(from:)` signature.
+
+protocol P {
+  associatedtype X
+}
+
+struct S: Codable {
+  var foo: String?
+
+  enum CodingKeys: CodingKey {
+    case foo
+  }
+
+  init<T: P>(from: T) where T.X == CodingKeys {}
+}