Make some NullablePointer enums FFI-compatible with the base pointer type.
auto merge of #14121 : luqmana/rust/option-ffi, r=nikomatsakis

This slightly adjusts the NullablePointer representation for some enums in the case where the non-nullable variant has a single field (the ptr field) to be just that, the pointer. This is in contrast to the current behaviour where we'd wrap that single pointer in a LLVM struct.

Fixes #11040 & #11303.