compiler: Better error message for invalid use of predeclared function.
compiler: Better error message for invalid use of predeclared function.

R=iant
CC=gofrontend-dev
https://golang.org/cl/5572051

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