Intl constructors are specified to prohibit structurally invalid
subtags. BCP 47 defines itself to be case-insensitive. Firefox does
throw on case-insensitive duplicates, following the specifications.
This patch makes V8 do the same. There is some small compatibility
risk, but the case is fairly niche, so I hope it does not cause
much breakage.

BUG=v8:4215

Review-Url: https://codereview.chromium.org/2639333003
Cr-Commit-Position: refs/heads/master@{#42487}