compiler: Don't assign to embedded builtins in imported struct composite literals.
compiler: Don't assign to embedded builtins in imported struct composite
literals.

Fixes https://github.com/golang/go/issues/6832.

LGTM=iant
R=iant
CC=gofrontend-dev
https://golang.org/cl/191500043

qskycolor/gofrontend,qskycolor/gofrontend,golang/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend