cmd/compile: use AutogeneratedPos for method value wrapper

We use AutogeneratedPos for most compiler-generated functions. But
for method value wrappers we currently don't. Instead, we use the
Pos for their (direct) declaration if there is one, otherwise
not set it in methodValueWrapper, which will probably cause it to
inherit from the caller, i.e. the Pos of that method value
expression. If that Pos has inline information, it will cause the
method wrapper to have bogus inline information, which could lead
to infinite loop when printing a stack trace.

Change it to use AutogeneratedPos instead.

Fixes #51401.

Change-Id: I398dfe85f9f875e1fd82dc2f489dab63ada6570d
Reviewed-on: https://go-review.googlesource.com/c/go/+/388794


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>