compiler: Add explicit sign to exported complex constants without imaginary component.
compiler: Add explicit sign to exported complex constants without imaginary component.

Fixes #31.

LGTM=iant
R=iant
CC=gofrontend-dev
https://golang.org/cl/186710043

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