IRGen: Default to clang's frame pointer elimination settings

Clang provides options to override that default value.
These options are accessible via the -Xcc flag.

Some Swift functions explicitly disable the frame pointer.

The clang options will not override those.

lib/Frontend/CompilerInvocation.cpp

@@ -1514,9 +1514,6 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
         getRuntimeCompatVersion();
   }
 
-  if (Args.hasArg(OPT_disable_leaf_frame_pointer_elim))
-    Opts.DisableFPElimLeaf = true;
-
   return false;
 }