HLSL: allow non-vec3 tessellation coordinate declarations

HLSL requires vec2 tessellation coordinate declarations in some cases
(e.g, isoline topology), where SPIR-V requires the TessCoord qualified
builtin to be a vec3 in all cases.  This alters the IO form of the
variable to be a vec3, which will be copied to the shader's declared
type if needed.  This is not a validation; the shader type must be correct.

Test/hlsl.domain.3.tese

@@ -0,0 +1,31 @@
+// Test vec2 tessellation coordinate: the IO form should be a vec3, copied to a vec2
+// at the entry point boundary.
+
+struct ds_in_t 
+{ 
+    float4 pos  : POSITION; 
+    float3 norm : TEXCOORD0; 
+}; 
+
+struct pcf_in_t 
+{ 
+    float flTessFactor [3]   : SV_TessFactor; 
+    float flInsideTessFactor : SV_InsideTessFactor; 
+}; 
+
+struct gs_in_t 
+{ 
+    float4 pos  : POSITION; 
+    float3 norm : TEXCOORD0; 
+}; 
+
+[domain ( "isoline" )]
+gs_in_t main (const OutputPatch <ds_in_t, 2> i, float2 tesscoord : SV_DomainLocation, pcf_in_t pcf_data ) 
+{ 
+    gs_in_t o; 
+
+    o.pos  = i[0].pos + tesscoord.x;
+    o.norm = i[0].norm + tesscoord.y;
+
+    return o; 
+}