R=go1.11

The terminating statement check for functions that declare result
parameters was using the wrong scope to look up calls to `panic`
which in esoteric cases lead to a false positive.

Instead of looking up a panic call again at a time when correct
scope information would have to be recomputed, collect calls to
predeclared panic in a set when type-checking that call.

Fixes #23218.

Change-Id: I35eaf010e5cb8e43696efba7d77cefffb6f3deb2
Reviewed-on: https://go-review.googlesource.com/85198Reviewed-by: Alan Donovan <adonovan@google.com>