Remove implicit conversions to `void` (#1388)

* 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

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/diagnostics/void-function-returning-value.slang.expected b/tests/diagnostics/void-function-returning-value.slang.expected
new file mode 100644
index 000000000..a94132971
--- /dev/null
+++ b/tests/diagnostics/void-function-returning-value.slang.expected
@@ -0,0 +1,7 @@
+result code = -1
+standard error = {
+tests/diagnostics/void-function-returning-value.slang(16): error 30019: expected an expression of type 'void', got 'int'
+tests/diagnostics/void-function-returning-value.slang(16): note: explicit conversion from 'int' to 'void' is possible
+}
+standard output = {
+}