[Runtime] Correctly match demangle tree for generic Objective-C classes.

When we encounter the demangle tree for a bound generic class type, look
through the "Type" node of the child tree before checking whether we
have an Objective-C class name. If we do have an Objective-C class name,
there is no way to preserve the generic arguments, so we ignore them and
return the (non-generic) class type.

Fixes rdar://problem/47028102.

test/Runtime/demangleToMetadataObjC.swift

@@ -108,5 +108,9 @@ DemangleToMetadataTests.test("runtime conformance check for @objc protocol inher
   expectEqual(F<P3>.self, _typeByName("4main1FCyAA2P3PG")!)
 }
 
+DemangleToMetadataTests.test("Objective-C generics") {
+  expectEqual(NSArray.self, _typeByName("So7NSArrayCySo8NSStringCG")!)
+}
+
 runAllTests()