Add conversion folding when the source is a constant.

This change adds unary conversion folding when the source is a constant.
This fixes an ISV issue whereby:

```
const float16_t f = float16_t(42.0);
```

Wouldn't compile because the conversion operator would always produce an
EvqTemporary when it could have produced an EvqConst.

I've also added a test case that proves out that all basic-type to
basic-type conversions work.

gtests/AST.FromFile.cpp

@@ -231,6 +231,7 @@ INSTANTIATE_TEST_CASE_P(
         "precise_struct_block.vert",
         "maxClipDistances.vert",
         "findFunction.frag",
+        "constantUnaryConversion.comp"
     })),
     FileNameAsCustomTestSuffix
 );