[GLSL ES 1.0.17] extension directives must occur before any non-preprocessor tokens

In page 15 of the GLSL ES 1.0.17 specification, it states "extension directives must occur before any non-preprocessor tokens".

The following shader should cause a compiler error:

```
#version 100
uniform mediump vec4 colour;
#extension GL_OES_standard_derivatives : enable
void main() {
gl_FragColor = colour;
}
```

In Glslang Version: 3.0.790 15-Oct-2015, an error isn't generated.

Use the supplied GLSL ES shader as the reproducer and compare the compiler's diagnostics with the specification's ordering rule. Done means the compiler rejects the shader because the #extension directive follows a uniform declaration; add regression coverage wherever the existing compiler tests cover this validation.

Written by the indexing model from the issue text.