[Feature Req] Allow preprocessing GLSL files without enabling GL_GOOGLE_cpp_style_line_directive

I'm using `glslc` to preprocess the same GLSL files targeting both SPIR-V and regular OpenGL 4.1 with a rather limited number of extensions.

The problem? OpenGL 4.1 on my target device isn't a particularly big fan of that extension or any `#line` directives in my source. It would be nice if there was a flag to force glslc to produce 'classic'-style #line directives instead of these custom ones. Perhaps `-no-cpp-style-line-directive`?