Warn about bogus comparisons

Today, someone filed a [bug against our Vulkan driver](https://bugs.freedesktop.org/show_bug.cgi?id=100134) saying that it hangs when you try to run the following compute shader compiled, I believe, with glslang:

```
#version 450
#extension GL_ARB_separate_shader_objects : enable

shared int roi[8];

void main() {
for (uint i = 7; i >= 0; i--) {
roi[i] = 1;
}
}
```

The problem with the shader was that they used an unsigned integer for `i` and a condition of `i >= 0` which always evaluates to true yielding an infinite loop. Since we do nothing in our Vulkan driver to guard against infinite loops (why would we?) they got a GPU hang.

It would be really nice if glslang could emit some sort of a warning when someone tries to do an unsafe comparison in a similar way to what GCC does with -Wsign-compare. If it had such a warning, they probably would never have even submitted the bug.

Start with the supplied GLSL compute shader and compare its unsigned `i >= 0` condition with GCC's `-Wsign-compare` behavior. Trace how glslang handles comparisons and diagnostics, then verify that an appropriate warning is emitted for this always-true comparison without changing valid shader compilation.

Written by the indexing model from the issue text.