Our current implementation of typeOf (class Typed in LLVM.AST.Typed) has a problem: if the type resolution logic encounters an opaque structure type, typeOf throws an error and bails.

This can happen any time someone asks for the type of something resulting from a getelementptr instruction.

This is problematic because it doesn't allow the user to gracefully handle the failure of type resolution. typeOf should really be returning a Maybe Type instead of returning a Type and throwing an error if that type can't be resolved.