cmd/compile: set correct line number for method wrappers

When converting a method to a function, like this:

type T ...
func (t T) foo() {
}
var t T
f := t.foo

We need to build a wrapper function for the partially evaluated
method. Currently that wrapper function gets the line number of
the first place where t.foo appears. Instead it should have the
line number of where foo is declared.

Fixes #26839

Change-Id: I7dbe2094e53d5d336f329273f10f8430e0af544e
Reviewed-on: https://go-review.googlesource.com/c/153498


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>