CMake: Fold HLSL source into glslang

CMake: Fold HLSL source into glslang

... 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