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* Remove implicit conversions to `void`
Fixes #1372
The standard library code had accidentally introduced implicit-conversion `__init` operations on the `void` type that accepted each of the other basic types, so that a function written like:
```hlsl
void bad() { return 1; }
```
would translate to:
```hlsl
void bad() { return (void)1; }
```
The dual problesm are that the input code should have produced a diagnostic of some kind, and the output code doesn't appear to compile correctly through fxc.
This change introduces several fixes aimed at this issue:
* First, the problem in the stdlib code is plugged: we don't introduce implicit conversion operations *to* or *from* `void` (we'd only been banning it in one direction before)
* Next, an explicit `__init` was added to `void` that accepts *any* type so that existing HLSL code that might do `(void) someExpression` to ignore a result will continue to work. This is a compatibility feature, and it might be argued that we should at least warn when it is used. Note that this function is expected to never appear in output HLSL/GLSL because its result will never be used, and it is marked `[__readNone]` allowing calls to it to be eliminated as dead code.
* During IR lowering, we now take care to only emit the `IRReturnVal` instruction type if there is a non-`void` value being returned, and use `IRReturnVoid` for both the case where no expression was used in the `return` statement *and* the case where an expression of type `void` is returned.
* A test case was added to confirm that returning `1` from a `void` function isn't allowed, while returning `(void) 1` *is*.
The net result of these changes is that we now produce an error for the bad input code, we allow explicit casts to `void` as a compatibility feature, and we are more robust about treating `void` as if it is an ordinary type in the front-end.
* fixup: missing file
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