`glslang_pch()` did manual mangling of the compiler flags to enable pre-compiled headers.
I couldn't get this approach to work with the `MachineIndependent` subdirectory, but fortunately CMake has added first-class support for precompiled headers in 3.16, which does work with subdirectories.
Moved `glslang_pch()` to the other global function declarations.
`glslang_pch()` is a no-op when using CMake earlier than `3.16`.
CMake's PCH implementation does not need the `pch.cpp` files, so just remove them.
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.
