Now that the HLSL source files are part of the `glslang` target (#2271), it makes sense for these to sit in the `glslang` directory.
Changed the case of the directory from `hlsl` to `HLSL` to better match the sibling directories.
In order to avoid potential build failures that could arise from
SPIRV-Tools/Headers, and to avoid reading known_good.json, this build
configuration does not build the SPIRV-Tools dependency and hence cannot
perform HLSL compilation to SPIR-V.
