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.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline and GCC 7 branch.

Ian