1. Change the function to align with UTS35 instead of (old) RFC.
2. Restrict tag as Unicode Locale Identifier by code that on top of ICU.
Because icu::Locale in ICU63 is not tight enough to restrict that.
Added comments to show what to do (to use icu::LocaleBuilder) after
the landing of ICU64
3. Fix to pass two tests in test262.

Bug: v8:8262,v8:8670,v8:7684
Change-Id: If244647e6a50daae305b9ae1a569100859be8a0c
Reviewed-on: https://chromium-review.googlesource.com/c/1435737
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59382}