Teach RAV to visit parameter variable declarations of implicit functions. Fixes PR16182.

Normally RAV visits parameter variable declarations of a function by traversing the TypeLoc of
the parameter declarations. However, for implicit functions, their parameters don't have any
TypeLoc, because they are implicit.

So for implicit functions, we visit their parameter variable declarations by traversing them through
the function declaration, and visit them accordingly.

Reviewed by Richard Smith and Manuel Klimek.