language: support en_US_POSIX

Also make grandfathered tags case-insensitve, as they should be and
support the legacy en_US_POSIX. The latter is widely used in CLDR
so not supporting it was kind of a pain.

Note that checking for grandfathered tags is in the critical path.

Change-Id: Ie635ecdf56e5e9de95c9c451bc7afb6985587d66
Reviewed-on: https://go-review.googlesource.com/14555
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed