`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.