Implement specialization constant support in numthreads / local_size

[juliusikkala]

Adds support for using SPIR-V specialization constants in numthreads:

[vk::constant_id(0)] const int WG_SIZE = 8;

[shader("compute")]
[numthreads(1, WG_SIZE, 1)]
void main(uint3 tid : SV_DispatchThreadID)
{
}

Similar to the local_size_x_id feature from GLSL, this allows users to generate shader permutations with different work group shapes / thread counts after the shader has already been compiled to SPIR-V.

Currently, this works with the SPIR-V (using LocalSizeId execution mode) and GLSL (using local_size_{x,y,z}_id) targets. I'm not sure if there even is anything that could be done for the other targets, other than to just present diagnostics about this feature not being supported there.

TODO (hence draft):

• GLSL compatibility syntax (fixes local_size_x_id layout qualifier is unrecognized in GLSL compatibility mode #5948)
• Proper testing of the SPIR-V output (I only checked the disassembly which looked OK)
• All related diagnostics (at least about parsing errors + unsupported targets)
• Fix compiler segfaults with some references to gl_WorkGroupSize