HLSL: implement numthreads for compute shaders

This PR adds handling of the numthreads attribute for compute shaders, as well as a general
infrastructure for returning attribute values from acceptAttributes, which may be needed in other
cases, e.g, unroll(x), or merely to know if some attribute without params was given.

A map of enum values from TAttributeType to TIntermAggregate nodes is built and returned.  It
can be queried with operator[] on the map.  In the future there may be a need to also handle
strings (e.g, for patchconstantfunc), and those can be easily added into the class if needed.

New test is in hlsl.numthreads.comp.

gtests/Hlsl.FromFile.cpp

@@ -150,6 +150,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.multiReturn.frag", "main"},
         {"hlsl.matrixindex.frag", "main"},
         {"hlsl.numericsuffixes.frag", "main"},
+        {"hlsl.numthreads.comp", "main_aux1"},
         {"hlsl.overload.frag", "PixelShaderFunction"},
         {"hlsl.pp.line.frag", "main"},
         {"hlsl.precise.frag", "main"},