Don't mark funciton argument struct names as declarations

[AndrewRayCode]

Fixes

• Issue doesn't handle fn(keyword structname arg) {} #3
• Doesn't descope struct name in fn args glsl-token-descope#3
• Can't parse user defined structs in fn args stackgl/glsl-parser#24

[AndrewRayCode]

From looking at https://www.khronos.org/registry/OpenGL/specs/gl/GLSLangSpec.1.20.pdf - I can't think of a better way to do this. We don't know the grammar/syntax types at this stage of lexing (nor does the parser ever know it) - what I'm not sure about is if there's some other user-defined ident that would cause this to fail