Fix another BCP 47 validation issue.

Duplicate U-ext keywords are allowed, but previously it's rejected.
Cherry-pick an upstream patch for that.
See https://github.com/unicode-org/icu/pull/136

TBR=gsathya@chromium.org
Bug: v8:8136
Test: v8 - test262/intl402/Locale/constructor-unicode-ext-valid
Change-Id: Iaec82e28e728afb09f5b1d6b63b1d72bedf02e47
Reviewed-on: https://chromium-review.googlesource.com/1238056
Reviewed-by: Jungshik Shin <jshin@chromium.org>
3 files changed