SILGen: Use super_method for native non-final methods in classes

Use the `super_method` instruction for non-final `func` and `class func`
declarations in native Swift classes. Previously, we would always emit
a static `function_ref` for these, which prevents resilient dynamic
dispatch.

This is hidden behind a -use-native-super-dispatch flag while I
survey the effects on devirtualization and stack promotion. When
that's figured out, I'll add more tests and update test cases that
still assume static dispatch.

rdar://problem/22749732

lib/Frontend/CompilerInvocation.cpp

@@ -1002,6 +1002,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
 
   Opts.GenerateProfile |= Args.hasArg(OPT_profile_generate);
   Opts.EmitProfileCoverageMapping |= Args.hasArg(OPT_profile_coverage_mapping);
+  Opts.UseNativeSuperMethod |=
+    Args.hasArg(OPT_use_native_super_method);
 
   return false;
 }