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.

gtests/CMakeLists.txt

@@ -60,9 +60,9 @@ if(BUILD_TESTING)
                 ${CMAKE_CURRENT_SOURCE_DIR}/Remap.FromFile.cpp)
         endif()
 
-        glslang_pch(TEST_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/pch.cpp)
 
         add_executable(glslangtests ${TEST_SOURCES})
+        glslang_pch(glslangtests ${CMAKE_CURRENT_SOURCE_DIR}/pch.h)
         set_property(TARGET glslangtests PROPERTY FOLDER tests)
         glslang_set_link_args(glslangtests)
         if(ENABLE_GLSLANG_INSTALL)