glslangValidator '-e' doesn't work correctly

I've got this shader code:

```glsl
#version 450

void mainVS()
{
gl_Position = vec4(0.0, 0.0, 0.0, 1.0);
}
```

and when I run

`glslangValidator -V -S vert -e mainVS -o vertex_shader.spv shader.glsl`

I get this error: `ERROR: Linking vertex stage: Missing entry point: Each stage requires one entry point`

when I change `void mainVS()` to `void main()` and run `glslangValidator...` again, it works fine.

Start by reproducing the reported command with the provided shader and compare it with the version using main(). Trace how glslangValidator handles the -e entry-point option for the vertex stage. Done means a named entry point such as mainVS is accepted and the shader produces vertex_shader.spv without the missing-entry-point error.

Written by the indexing model from the issue text.