HLSL: Improve setting and testing of interpolation qualifiers.

Notably, use of 'linear' on a non-input could mark it as an input.

Test/hlsl.structin.vert

@@ -1,9 +1,14 @@
 struct VI {
     float4 m[2];
     uint2 coord;
+    linear float4 b;
 };
 
-float4 main(float4 d, VI vi, float4 e) : SV_POSITION
+VI main(float4 d, VI vi, float4 e) : SV_POSITION
 {
-    return vi.m[1] + vi.m[0] + float4(vi.coord.x) + d + e;
+    VI local;
+
+    local.b = vi.m[1] + vi.m[0] + float4(vi.coord.x) + d + e;
+
+    return local;
 }