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/baseResults/hlsl.numthreads.comp.out

@@ -0,0 +1,60 @@
+hlsl.numthreads.comp
+Shader version: 450
+local_size = (4, 4, 2)
+0:? Sequence
+0:4  Function Definition: main(vu3; (temp void)
+0:4    Function Parameters: 
+0:4      'tid' (in 3-component vector of uint)
+0:9  Function Definition: main_aux1(vu3; (temp void)
+0:9    Function Parameters: 
+0:9      'tid' (in 3-component vector of uint LocalInvocationID)
+0:?   Linker Objects
+0:?     'tid' (in 3-component vector of uint LocalInvocationID)
+
+
+Linked compute stage:
+
+
+Shader version: 450
+local_size = (4, 4, 2)
+0:? Sequence
+0:4  Function Definition: main(vu3; (temp void)
+0:4    Function Parameters: 
+0:4      'tid' (in 3-component vector of uint)
+0:9  Function Definition: main_aux1(vu3; (temp void)
+0:9    Function Parameters: 
+0:9      'tid' (in 3-component vector of uint LocalInvocationID)
+0:?   Linker Objects
+0:?     'tid' (in 3-component vector of uint LocalInvocationID)
+
+// Module Version 10000
+// Generated by (magic number): 80001
+// Id's are bound by 15
+
+                              Capability Shader
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint GLCompute 4  "main_aux1" 14
+                              ExecutionMode 4 LocalSize 4 4 2
+                              Name 4  "main_aux1"
+                              Name 11  "main(vu3;"
+                              Name 10  "tid"
+                              Name 14  "tid"
+                              Decorate 14(tid) BuiltIn LocalInvocationId
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeInt 32 0
+               7:             TypeVector 6(int) 3
+               8:             TypePointer Function 7(ivec3)
+               9:             TypeFunction 2 8(ptr)
+              13:             TypePointer Input 7(ivec3)
+         14(tid):     13(ptr) Variable Input
+    4(main_aux1):           2 Function None 3
+               5:             Label
+                              Return
+                              FunctionEnd
+   11(main(vu3;):           2 Function None 9
+         10(tid):      8(ptr) FunctionParameter
+              12:             Label
+                              Return
+                              FunctionEnd