Track stage for varying sub-fields (#842)

Fixes #841

This reverts a small change made in #815 that seemed innocent at the time: we stopped tracking an explicit `Stage` to go with every `VarLayout` that is part of an entry-point varying parameter, and instead only associated the stage with the top-level parameter. That change ended up breaking the logic to emit the `flat` modifier automatically for integer type fragment-shader inputs for GLSL, but we didn't have a regression test to catch that case.

This change adds a regression test to cover this case, and adds the small number of lines that were removed from `parameter-binding.cpp`.
A few other test outputs had to be updated for the change (these are outputs that were changed in #815 for the same reason).

2 files changed, 54 insertions, 0 deletions

diff --git a/tests/bugs/gh-841.slang b/tests/bugs/gh-841.slang
new file mode 100644
index 000000000..becf741c1
--- /dev/null
+++ b/tests/bugs/gh-841.slang
@@ -0,0 +1,19 @@
+// gh-841.slang
+
+//TEST:CROSS_COMPILE: -profile ps_5_0 -entry main -target spirv-assembly
+
+// GitHub issue #841: failing to emit `flat` modifier in output GLSL when required
+
+struct RasterVertex
+{
+	float4 c : COLOR;
+	uint u : FLAGS;
+};
+
+float4 main(RasterVertex v) : SV_Target
+{
+	float4 result = v.c;
+	if(v.u & 1)
+	    result += 1.0;
+   return result;
+}
diff --git a/tests/bugs/gh-841.slang.glsl b/tests/bugs/gh-841.slang.glsl
new file mode 100644
index 000000000..caee80928
--- /dev/null
+++ b/tests/bugs/gh-841.slang.glsl
@@ -0,0 +1,35 @@
+//TEST_IGNORE_FILE:
+#version 450
+
+layout(location = 0)
+out vec4 _S1;
+
+layout(location = 0)
+in vec4 _S2;
+
+flat layout(location = 1)
+in uint _S3;
+
+struct RasterVertex_0
+{
+    vec4 c_0;
+    uint u_0;
+};
+
+void main()
+{
+    vec4 result_0;
+    RasterVertex_0 _S4 = RasterVertex_0(_S2, _S3);
+    vec4 result_1 = _S4.c_0;
+
+    if(bool(_S4.u_0 & uint(1)))
+    {
+        result_0 = result_1 + 1.0;
+    }
+    else
+    {
+        result_0 = result_1;
+    }
+    _S1 = result_0;
+    return;
+}