Includes issue with Rust Nightly

[Eliah-Lakhin]

In Vulkano project(that is using shaderc-rs under the hood) we faced a false-positive glsl error happening with includes feature on Rust Nightly build("rustc 1.49.0-nightly (91a79fb29 2020-10-07)").

This is a minimal reproducible example:

#version 450
#include <common.glsl>

Error messages from the Compile::compile_into_spirv function are:

           shader.glsl:2: error: '#line' : must by followed by an integral literal
           shader.glsl:2: error: 'preprocessor evaluation' : bad expression
           shader.glsl:2: error: '#line' : unexpected tokens following directive

The "common.glsl" file is properly configured through the CompileOptions::set_include_callback function. Neither the snippet above nor "common.glsl" contain a "#line" directive. And this test example works fine with Rust Stable.