go/types: don't clone interface methods when embedding them
https://golang.org/cl/191257 significantly changed (and simplified)
the computation of interface method sets with embedded interfaces.
Specifically, when adding methods from an embedded interface, those
method objects (Func Objects) were cloned so that they could have a
different source position (the embedding position rather than the
original method position) for better error messages.
This causes problems for code that depends on the identity of method
objects that represent the same method, embedded or not.
This CL avoids the cloning. Instead, while computing the method set
of an interface, a position map is carried along that tracks
embedding positions. The map is not needed anymore after type-
checking.
Updates #34421.
Change-Id: I8ce188136c76fa70fba686711167db29a049f46d
Reviewed-on: https://go-review.googlesource.com/c/go/+/196561 Reviewed-by: Matthew Dempsky <mdempsky@google.com>