Use type-alignment-sized integer for discriminant types

The previous behaviour of using the smallest type possible caused LLVM
to treat padding too conservatively, causing poor codegen. This commit
changes the behaviour to use an type-alignment-sized integer as the
discriminant. This keeps types the same size, but helps LLVM understand
the data structure a little better, resulting in better codegen.
Add enum size cases to type-sizes test
Make alignment-forcing case work the same on 64 and 32-bit platforms
auto merge of #20060 : Aatch/rust/enum-repr, r=alexcrichton

The previous behaviour of using the smallest type possible caused LLVM
to treat padding too conservatively, causing poor codegen. This commit
changes the behaviour to use an alignment-sized integer as the
discriminant. This keeps types the same size, but helps LLVM understand
the data structure a little better, resulting in better codegen.