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-1.frag

@@ -0,0 +1,6 @@
+float4 main(in float4 pos : SV_Position, 
+            in float clip : SV_ClipDistance,
+            in float cull : SV_CullDistance) : SV_Target0
+{
+    return pos + clip + cull;
+}