Previously, n.Pos was reassigned to lineno when declare was called,
which might not match where the identifier actually appeared in the
source. This caused a loss of position precision for function
parameters (which were all declared at the last parameter's position),
and required some clumsy workarounds in bimport.go.

This CL changes declare to leave n.Pos alone and also fixes a few
places where n.Pos was not being set correctly.

Change-Id: Ibe5b5fd30609c684367207df701f9a1bfa82867f
Reviewed-on: https://go-review.googlesource.com/104275Reviewed-by: Robert Griesemer <gri@golang.org>