Use CMake's builtin functionality for PCHs

`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.

BUILD.bazel

@@ -72,9 +72,7 @@ cc_library(
             "glslang/MachineIndependent/preprocessor/*.cpp",
         ],
         exclude = [
-            "glslang/HLSL/pch.cpp",
             "glslang/HLSL/pch.h",
-            "glslang/MachineIndependent/pch.cpp",
             "glslang/MachineIndependent/pch.h",
         ],
     ) + [