Go 1.8 includes a language change (https://golang.org/doc/go1.8#language):
in an explicit conversion from one struct type to another, any field
tags are ignored.
This CL implements this language change in the gofrontend. The tests
for this are in the gc testsuite, which will be copied into the gccgo
repository in due course.
Updates golang/go#16085.
Reviewed-on: https://go-review.googlesource.com/43614
From-SVN: r248248
