HLSL: allow non-vec3 tessellation coordinate declarations

HLSL requires vec2 tessellation coordinate declarations in some cases
(e.g, isoline topology), where SPIR-V requires the TessCoord qualified
builtin to be a vec3 in all cases.  This alters the IO form of the
variable to be a vec3, which will be copied to the shader's declared
type if needed.  This is not a validation; the shader type must be correct.

gtests/Hlsl.FromFile.cpp

@@ -125,6 +125,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.getsampleposition.dx10.frag", "main"},
         {"hlsl.domain.1.tese", "main"},
         {"hlsl.domain.2.tese", "main"},
+        {"hlsl.domain.3.tese", "main"},
         {"hlsl.hull.1.tesc", "main"},
         {"hlsl.hull.2.tesc", "main"},
         {"hlsl.hull.void.tesc", "main"},