go/gcimporter: update to latest copy from master
go/gcimporter: update to latest copy from master

Backport of CL 41619.
Generated by copying bimport.go and reverting
the chunk containing the "This file is a copy"
comment near the top.

Fixes golang/go#20121

Change-Id: If24c97d01a550318ab919c37cd0c4a8f34d776c7
Reviewed-on: https://go-review.googlesource.com/41756
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools