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.

hlsl/CMakeLists.txt

@@ -1,4 +1,5 @@
 set(SOURCES
+    hlslAttributes.cpp
     hlslParseHelper.cpp
     hlslScanContext.cpp
     hlslOpMap.cpp
@@ -6,7 +7,8 @@ set(SOURCES
     hlslGrammar.cpp
     hlslParseables.cpp)
 
-set(HEADERS
+  set(HEADERS
+    hlslAttributes.h
     hlslParseHelper.h
     hlslTokens.h
     hlslScanContext.h