HLSL: handle clip and cull distance input builtin type conversion

HLSL allows a range of types for clip and cull distances.  There are
three dimensions, including arrayness, vectorness, and semantic ID.
SPIR-V requires clip and cull distance be a single array of floats in
all cases.

This code provides input side conversion between the SPIR-V form and
the HLSL form.  (Output conversion was added in PR #947 and #997).

This PR extends HlslParseContext::assignClipCullDistance to cope with
the input side conversion.  Not as much changed as appears: there was
also a lot of renaming to reflect the fact that the code now handles
either direction.

Currently, non-{frag,vert} stages are not handled, and are explicitly
rejected.

Fixes #1026.

Test/hlsl.clipdistance-8.frag

@@ -0,0 +1,10 @@
+struct VS_OUTPUT        {
+    float4 Position             : SV_Position;
+    float3 clip0                : SV_ClipDistance0;  // multiple semantic IDs, vec3+float (pack)
+    float  clip1                : SV_ClipDistance1;  // ...
+};
+
+float4 main(VS_OUTPUT v) : SV_Target0
+{
+    return v.Position + v.clip0.x + v.clip1;
+}