GL_EXT_mesh_shader: clarify if interpolation qualifier can be used for per-primitive input/output

I can't find explicit words in spec for this topic. Interpolation qualifier (flat, smooth...) is not allowed to be used with "patch" qualifier in 4.6 spec:
```
The auxiliary storage qualifier patch is not used for interpolation; it is a compile-time error to use
interpolation qualifiers with patch.
```
So I expect interpolation qualifier should not be used for per-primitive IO as it makes no sense to interpolate per-primitive IO. But glslang compiler allow this usage (no compiler error).

Read the GL_EXT_mesh_shader specification together with the GLSL 4.6 wording for interpolation and patch qualifiers. Check how per-primitive input/output is defined and compare that rule with glslang's accepted usage. Done means the specification explicitly states whether interpolation qualifiers are permitted for per-primitive I/O.

Written by the indexing model from the issue text.