Add glsl barrier function

[fintelia]

This attempts to implement the GLSL barrier function based some of the surrounding builtin implementations. However, it panics later when emitting expressions, so it evidentially isn't correct yet.

Fixes #1771

[JCapucho]

Can you show an example that panics? I've tried to reproduce it on your branch with a very simple compute shader

#version 460 core

void main() {
    barrier();
}

But that works

[fintelia]

Two barriers in a row seems to be enough to cause a panic:

#version 460 core

void main() {
    barrier();
    barrier();
}

[kvark]

Would be nice to test it as well