CodeGen: Compound literals with funny types shouldn't crash

CodeGen assumed that a compound literal with array type should have a
corresponding LLVM IR array type.

We had two bugs in this area:
- Zero sized arrays in compound literals would lead to the creation of
  an opaque type.  This is unnecessary, we should just create an array
  type with a bound of zero.
- Funny record types (like unions) lead to exotic IR types for compound
  literals.  In this case, CodeGen must be prepared to deal with the
  possibility that it might not have an array IR type.

This fixes PR21912.