[SPIR-V] Add error for casting from vector to scalar

[cassiebeckley]

Fixes #4640

[vettoreldaniele]

Looks good, but I believe DXIL only outputs a warning and extends the variable with zero values.

Do we want to match DXIL's behavior here?

I also get two errors from this - is it intended?

../tools/clang/test/CodeGenSPIRV/cast.scalar-to-vec.implicit.hlsl:15:7: error: casting from 'float' to 'float3' unsupported
  foo(bar);
      ^
../tools/clang/test/CodeGenSPIRV/cast.scalar-to-vec.implicit.hlsl:15:7: error: casting from 'float3' to 'float' unsupported

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.2082 completed (commit bfdc9c2312 by @cassiebeckley)

[vettoreldaniele]

Also, I just noticed this fix only applies to float. But this behavior seems to be independent of type (see https://shader-playground.timjones.io/f1b2e245672750fae10053e504f16e22 for an example with int).

[s-perron]

#5249 will make this obsolete.