ES3.0 glslang frontend didn't recognize reserved keyword. image1DShadow

From sawato shusaku:

```
#version 300 es
precision mediump float;
in highp vec4 dEQP_Position;
void main()
{
float image1DShadow = 1.0;
gl_Position = dEQP_Position;
}

// expected behavior is compile failure.
// but glslang doesn't
```

keywords wrong is image1DShadow image2DShadow image1DArrayShadow image2DArrayShadow

Start in the glslang frontend by locating how reserved keywords are defined and checked for ES 3.0. Use the shader shown in the issue, plus variants for image1DShadow, image2DShadow, image1DArrayShadow, and image2DArrayShadow, and confirm compilation fails for each reserved identifier.

Written by the indexing model from the issue text.