Add tests showing we don't emit interpolation qualifiers in vertex shaders.

In MSL, these only have an effect on fragment `[[stage_in]]` members.
They have no effect in vertex shaders. The Khronos front end doesn't
even emit the SPIR-V decorations for them.

reference/opt/shaders-msl/vert/interpolation-qualifiers.vert

@@ -0,0 +1,36 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float2 v0 [[user(locn0)]];
+    float2 v1 [[user(locn1)]];
+    float3 v2 [[user(locn2)]];
+    float4 v3 [[user(locn3)]];
+    float v4 [[user(locn4)]];
+    float v5 [[user(locn5)]];
+    float v6 [[user(locn6)]];
+    float4 gl_Position [[position]];
+};
+
+struct main0_in
+{
+    float4 Position [[attribute(0)]];
+};
+
+vertex main0_out main0(main0_in in [[stage_in]])
+{
+    main0_out out = {};
+    out.v0 = in.Position.xy;
+    out.v1 = in.Position.zw;
+    out.v2 = float3(in.Position.x, in.Position.z * in.Position.y, in.Position.x);
+    out.v3 = in.Position.xxyy;
+    out.v4 = in.Position.w;
+    out.v5 = in.Position.y;
+    out.v6 = in.Position.x * in.Position.w;
+    out.gl_Position = in.Position;
+    return out;
+}
+

reference/shaders-msl/vert/interpolation-qualifiers.vert

@@ -0,0 +1,36 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float2 v0 [[user(locn0)]];
+    float2 v1 [[user(locn1)]];
+    float3 v2 [[user(locn2)]];
+    float4 v3 [[user(locn3)]];
+    float v4 [[user(locn4)]];
+    float v5 [[user(locn5)]];
+    float v6 [[user(locn6)]];
+    float4 gl_Position [[position]];
+};
+
+struct main0_in
+{
+    float4 Position [[attribute(0)]];
+};
+
+vertex main0_out main0(main0_in in [[stage_in]])
+{
+    main0_out out = {};
+    out.v0 = in.Position.xy;
+    out.v1 = in.Position.zw;
+    out.v2 = float3(in.Position.x, in.Position.z * in.Position.y, in.Position.x);
+    out.v3 = in.Position.xxyy;
+    out.v4 = in.Position.w;
+    out.v5 = in.Position.y;
+    out.v6 = in.Position.x * in.Position.w;
+    out.gl_Position = in.Position;
+    return out;
+}
+

shaders-msl/vert/interpolation-qualifiers.vert

@@ -0,0 +1,22 @@
+#version 450
+
+layout(location=0) in vec4 Position;
+
+layout(location=0) out vec2 v0;
+layout(location=1) out noperspective vec2 v1;
+layout(location=2) out centroid vec3 v2;
+layout(location=3) out centroid noperspective vec4 v3;
+layout(location=4) out sample float v4;
+layout(location=5) out sample noperspective float v5;
+layout(location=6) out flat float v6;
+
+void main() {
+	v0 = Position.xy;
+	v1 = Position.zw;
+	v2 = vec3(Position.x, Position.z * Position.y, Position.x);
+	v3 = Position.xxyy;
+	v4 = Position.w;
+	v5 = Position.y;
+	v6 = Position.x * Position.w;
+	gl_Position = Position;
+}