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.
This adds solution folders that properly group gtest/glslang/hlsl.
This also marks gtest options as advanced so they don't show up
in cmake-gui by default.
