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-2.geom

@@ -0,0 +1,19 @@
+struct S {
+    float4 pos     : SV_Position;
+    float2 clip[2] : SV_ClipDistance0;
+};
+
+[maxvertexcount(3)]
+void main(triangle in float4 pos[3] : SV_Position, 
+          triangle in uint VertexID[3] : VertexID,
+          inout LineStream<S> OutputStream,
+          triangle in float2 clip[3][2] : SV_ClipDistance)  // scalar float
+{
+    S s;
+
+    s.pos = pos[0];
+    s.clip[0] = clip[0][0];
+    s.clip[1] = clip[0][1];
+
+    OutputStream.Append(s);
+}