Implement fixes for constant unions

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 #6100
Fix #6699
Fix #6895
Fix #6896
Fix #6897
Fix #7036
Fix #7083
Fix #7091

src/llvm_backend.cpp

@@ -3439,7 +3439,7 @@ gb_internal bool lb_generate_code(lbGenerator *gen) {
 						cc.link_section = e->Variable.link_section;
 
 						ExactValue v = tav.value;
-						lbValue init = lb_const_value(m, e->type, v, lb_build_expr_original_const_type(decl->init_expr), cc);
+						lbValue init = lb_const_value(m, e->type, v, cc);
 
 
 						LLVMDeleteGlobal(g.value);