* Sema: fix returned operands not coercing to the function return type
in some cases.
- When returning an error or an error union from a function with an
inferred error set, it will now populate the inferred error set.
- Implement error set coercion for the common case of inferred error
set to inferred error set, without forcing a full resolution.
* LLVM backend: update instruction lowering that handles error unions
to respect `isByRef`.
- Also implement `wrap_err_union_err`.
6534f2ef4f