Construct a constant value, suitable for initializing a GlobalVariable, given a case and constant values for its fields. Note that this may have a different LLVM type (and different alignment!) from the representation's type_of, so it needs a pointer cast before use.

The LLVM type system does not directly support unions, and only pointers can be bitcast, so a constant (and, by extension, the GlobalVariable initialized by it) will have a type that can vary depending on which case of an enum it is.

To understand the alignment situation, consider enum E { V64(u64), V32(u32, u32) } on win32. The type has 8-byte alignment to accommodate the u64, but V32(x, y) would have LLVM type {i32, i32, i32}, which is 4-byte aligned.

Currently the returned value has the same size as the type, but this could be changed in the future to avoid allocating unnecessary space after values of shorter-than-maximum cases.