Flow with non-trivial access path through receiver of method assigned to a variable is missed

Most likely some logic is missing to trace a call to a variable back to the possible callees before looking for the receiver.

This is demonstrated in some tests that have been marked `MISSING` in `ql/test/library-tests/semmle/go/dataflow/GenericFunctionsAndTypes/generictypesandmethods.go`, which will be introduced in https://github.com/github/codeql-go/pull/686 .