[dev.typeparams] cmd/compile: set sym.Def to ir.Name for method value wrappers

The code for generating method value wrappers is weird that it sets
sym.Def to the generated ir.Func, whereas normally sym.Def points to
ir.Name.

While here, change methodValueWrapper to return the ir.Name too, since
that's what the caller wants.

Change-Id: I3da5320ca0bf4d32d7b420345454f19075d19a26
Reviewed-on: https://go-review.googlesource.com/c/go/+/339410


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>