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.

Test/hlsl.numthreads.comp

@@ -0,0 +1,14 @@
+
+[numthreads(8,8,1)]
+void main(uint3 tid : SV_DispatchThreadID )
+{
+}
+
+[numTHreaDs(4,4,2)]  // case insensitive
+void main_aux1(uint3 tid : SV_DispatchThreadID )
+{
+}
+
+[numthreads(1,4,8)]
+void main_aux2(uint3 tid : SV_DispatchThreadID );
+