go/types: fix scope extents for range and type switch variables

The changes match the existing compilers, and assume an adjusted
spec (per issue #16794).

Fixes #15686.

Change-Id: I72677ce75888c41a8f3c2963117a2f2d5501c42b
Reviewed-on: https://go-review.googlesource.com/27290


Reviewed-by: Matthew Dempsky <mdempsky@google.com>