IRGen: Start a type verifier to fuzz static type info against runtime type info.

We've had a rash of bugs due to inconsistencies between how IRGen and the runtime think types are laid out. Add a '-verify-type-layout' mode to the frontend that causes IRGen to emit a bunch of code that compares its static assumptions against what the runtime value witness does.

Swift SVN r24918

lib/Frontend/CompilerInvocation.cpp

@@ -932,6 +932,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_use_jit))
     Opts.UseJIT = true;
   
+  if (Args.hasArg(OPT_verify_type_layout))
+    Opts.VerifyTypeLayout = true;
+  
   return false;
 }