Note that with this commit, casting through
multiply-nested unions is forbidden, e.g.
```
U :: union {int, V}
V :: union {bool}
x: U = true
```
does not compile.
(Previously the compiler would simply crash)
This is to avoid situations where adding variants
can lead to unexpected changes in the value.
For example if `U` is changed to have a `bool`
variant of its own:
```
U :: union {int, bool, V}
```
Then `x: U = true` would equal
`U(true)` instead of `U(V(true))`.
Single-variant unions are exempt, primarily to
improve the ergonomics of `Maybe` in cases like:
```
x: Maybe(union{int, bool}) = 1
```
Fix#6100Fix#6699Fix#6895Fix#6896Fix#6897Fix#7036Fix#7083Fix#7091
* Emit label debug info w/o location
* Insert debug label call
* Slight refactor for later fix
* Improve debug labels for block statements
* Improve debug info with for loops
* Generate label lbBlocks w/ debug
* Lightly refactor lb_add_debug_label
* Revise comments, add null check assertion
* Use LLVM-C API for debug labels
* Prefer C DILabel API for POSIX, fallback to CPP
* Use version check for LLVM-C DILabel
This can be important if matrices or SIMD vectors are being used in
global or static variables, as otherwise it may result in crashes due to
aligned instructions accessing misaligned variables.
If |result_count| is 0 then |results| will be a nullptr and hence the
access |results->Tuple| is undefined behaviour. There's already an
early return in the 0 branch so move that to be the first thing so that
we can guarantee that it's not a nullptr.
Note that technically we take the address of the result so it's not
actually dereferencing it, however UBSan doesn't care about that.
