compiler: Report unused variables initialized to function literals.
compiler: Report unused variables initialized to function literals.

Fixes golang/go#12317.

Change-Id: Ida147dbd6797b4a458c4d612a5198f4e7860a8f1
Reviewed-on: https://go-review.googlesource.com/13908
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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