compiler: Improve handling of invalid ASCII characters in identifiers.
compiler: Improve handling of invalid ASCII characters in identifiers.

R=iant
CC=gofrontend-dev
https://golang.org/cl/14512055

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