HLSL: various SPIR-V compute shader IDs must be a 3-vector of integers.

This PR forces the external definition of SV_GroupID variables to 3-vectors.
The conversion process between the shader-declared type and the external type
happens in wrapped main IO variable conversion.

The same applies to SV_DispatchThreadID and SV_GroupThreadID.

Fixes: #1371

Test/hlsl.groupid.comp

@@ -0,0 +1,9 @@
+RWTexture2D < float4 > OutputTexture;
+
+// Test conversion between SPIR-V required uint3 group id, and sub-vec3 shader declaration.
+
+[ numthreads ( 8 , 8 , 1 ) ] 
+void main ( uint2 vGroupId : SV_GroupID ) 
+{ 
+    OutputTexture[ vGroupId . xy ] = float4(1.0, 0.0, 0.0, 1.0); 
+}