go/types: set correct Pos for T in struct{p.T}

go/types: set correct Pos for T in struct{p.T}

Previously, the field Var for T created for struct{p.T}
would use the Pos of the ast.Field, which coincides with p.
This change makes it use the Pos of T.

Errors about the field type are still reported at the
position of the ast.Field (e.g. *p.T) not the field T.

Fixes #60372

Change-Id: I06000874f2018d47159493626da3d16e6716f4c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/497882
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>