Fix to allow using the glslang_shader_parse from C API without preprocess

[antaalt]

Hello,

I went into problem with the C API of glslang. Here is a small repro sample of the issue:

#version 450
#extension GL_EXT_debug_printf : require
void main() {
    debugPrintfEXT("global thread id %v3u\n", gl_GlobalInvocationID);
}

// Simple C code to run 
glslang_shader_t* shader = glslang_shader_create(&shader_input);
glslang_shader_preprocess(shader, &shader_input);
glslang_shader_parse(shader.handle.as_ptr(), &shader_input);

This will produce an End of line in string error for \n within string, but glslangValidator does not produce this error. After digging into it, it seems calling preprocess will replace \n string by a newline feed character (Code at MachineIndependant/preprocessor/PpScanner.cpp:1136), which will then be used by parse & produce this error.

With this CL, I try to fix this by not forcing the parse function to have preprocess executed before. Right now the C API dont allow parsing without preprocessing while the C++ API allow it. This issue should probably be solved more deeply to allow preprocess & parsing aswell but as its a very minor issue, I dont think it should really be a problem.