[Mangled name -> metadata] Handle import-as-member types.

When import-as-member takes a C type and imports it as a nested type,
we end up with a nominal type descriptor for a nested type, but the
mangled name remains "flat". Cope with inconsistency to allow
_typeByMangledName() to handle such nested types.

test/Runtime/demangleToMetadataObjC.swift

@@ -60,12 +60,12 @@ DemangleToMetadataTests.test("Imported swift_wrapper types") {
     _typeByMangledName("So21NSURLFileResourceTypea")!)
 }
 
+// FIXME: Shouldn't need this?
 extension URLSessionTask.State: P2 { }
 
 DemangleToMetadataTests.test("Imported enum types") {
-  // FIXME: Should work
-  // expectEqual(NSURLSessionTask.State.self,
-  //  _typeByMangledName("So21NSURLSessionTaskStateV")!)
+  expectEqual(NSURLSessionTask.State.self,
+    _typeByMangledName("So21NSURLSessionTaskStateV")!)
 }
 
 runAllTests()