Fix subgroup support for ray tracing

Closes #1735

GlslangToSpv.cpp
- minor formatting cleanup

BaseTypes.h
- minor formatting cleanup
- add subgroup builtins to GetBuiltInVariableString
  (was resulting in "unknown built-in variable" messages in test output)

Initialize.cpp
- better naming and re-use of strings for subgroup builtin variable declarations
- define subgroup builtin variables in ray-tracing shaders

intermOut.cpp
- add handling of the EOpSubgroupParition* variables
  (was resulting in "ERROR: Bad aggregation op" messages in test output)

Update test results.
diff --git a/Test/baseResults/glsl.450.subgroupArithmetic.comp.out b/Test/baseResults/glsl.450.subgroupArithmetic.comp.out
index 6c91bb0..431423f 100644
--- a/Test/baseResults/glsl.450.subgroupArithmetic.comp.out
+++ b/Test/baseResults/glsl.450.subgroupArithmetic.comp.out
@@ -12,8 +12,8 @@
 0:17          'invocation' ( temp uint)
 0:17          mod ( temp uint)
 0:17            add ( temp uint)
-0:17              'gl_SubgroupInvocationID' ( in uint unknown built-in variable)
-0:17              'gl_SubgroupSize' ( in uint unknown built-in variable)
+0:17              'gl_SubgroupInvocationID' ( in uint SubgroupInvocationID)
+0:17              'gl_SubgroupSize' ( in uint SubgroupSize)
 0:17            Constant:
 0:17              4 (const uint)
 0:19      move second child to first child ( temp float)
@@ -7288,8 +7288,8 @@
 0:17          'invocation' ( temp uint)
 0:17          mod ( temp uint)
 0:17            add ( temp uint)
-0:17              'gl_SubgroupInvocationID' ( in uint unknown built-in variable)
-0:17              'gl_SubgroupSize' ( in uint unknown built-in variable)
+0:17              'gl_SubgroupInvocationID' ( in uint SubgroupInvocationID)
+0:17              'gl_SubgroupSize' ( in uint SubgroupSize)
 0:17            Constant:
 0:17              4 (const uint)
 0:19      move second child to first child ( temp float)