HLSL: Refactor attribute implementation.

- make it sharable with GLSL
- correct the case insensitivity
- remove the map; queries are not needed, all entries need processing
- make it easier to build bottom up (will help GLSL parsing)
- support semantic checking and reporting
- allow front-end dependent semantics and attribute name mapping

gtests/Hlsl.FromFile.cpp

@@ -247,7 +247,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.matrixindex.frag", "main"},
         {"hlsl.nonstaticMemberFunction.frag", "main"},
         {"hlsl.numericsuffixes.frag", "main"},
-        {"hlsl.numthreads.comp", "main_aux1"},
+        {"hlsl.numthreads.comp", "main_aux2"},
         {"hlsl.overload.frag", "PixelShaderFunction"},
         {"hlsl.opaque-type-bug.frag", "main"},
         {"hlsl.params.default.frag", "main"},