Better handling of undef values in llvm codegen

[dgrove-oss]

I (re)discovered today that the UndefinedLiteral doesn't quite do what its header comment says:

The undefined value. Usage of an undefined value results in a compilation error.

If we interpret a reference to UndefinedLiteral, we get a compilation error. -- Good.

But if we execute generated code that reads an llvm undef literal we don't trap...we just execute something with ill-defined semantics. In particular, I had messed up a header file for a constant used in an ioctl and we were just merrily executing it, getting back a -1 because the constant wasn't valid and blundering along to fail later. -- Not good, as it took a while to realize what was happening.

It would be much better if we could instead generated a trap or something that would cause a more obvious failure if it was actually reached at runtime.