[Runtime] Skip Objective-C type records when looking for types.

When we scan the type metadata records or conformances to look
for a type by name, skip over indirect Objective-C class
references. We won’t find anything new there, but we’ll 
currently crash if they exist.

test/Runtime/demangleToMetadataObjC.swift

@@ -36,5 +36,9 @@ DemangleToMetadataTests.test("Objective-C protocols") {
   expectEqual(type(of: f1_composition_NSCoding), _typeByMangledName("yySo8NSCoding_pc")!)
 }
 
+DemangleToMetadataTests.test("Classes that don't exist") {
+  expectNil(_typeByMangledName("4main4BoomC"))
+}
+
 runAllTests()