[analyzer] Use explicit call description mode in iterator checkers (#88913)

This commit explicitly specifies the matching mode (C library function,
any non-method function, or C++ method) for the `CallDescription`s
constructed in the iterator/container checkers.

This change won't cause major functional changes, but isn't NFC because
it ensures that e.g. call descriptions for a non-method function won't
accidentally match a method that has the same name.

Separate commits will perform (or have already performed) this change in
other checkers. My goal is to ensure that the call description mode is
always explicitly specified and eliminate (or strongly restrict) the
vague "may be either a method or a simple function" mode that's the
current default.

I'm handling the iterator checkers in this separate commit because
they're infamously complex; but I don't expect any trouble because this
transition doesn't interact with the "central" logic of iterator
handling.