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.
... and stub the HLSL target.
Fixes the building of shared libraries.
This breaks the cyclic dependency between the `glslang` and `hlsl` targets (by essentially removing the `hlsl` target).
The `BUILD.gn` and `BUILD.bazel` build rules already pull the `HLSL` source into the `glslang` target.
`Android.mk` is the only remaining build config that has a dedicated `HLSL` target, but this is explicity static and does not suffer the same link-time issues with the cyclic dependency (we may wish to stub this target too).
Related issue: #1484, #2147
Related PR: #2267
