Handle all isolation checking for function calls in one place
Isolation checking for calls had two separate implementation places:
one that looked at the declaration being called (for member
declarations) and one that worked on the actual call expression. Unify
on the latter implementation, which is more general and has access to
the specific call arguments. Improve diagnostics here somewher so we
don't regress in that area.
This refactoring shouldn't change the actual semantics, but it makes
upcoming semantic changes easier.
