Other implementations unconditionally initialize [[pattern12]] from
[[pattern]] regardless of whether [[pattern]] has an hour pattern of h11
or h12. LibUnicode does not do this. So when InitializeDateTimeFormat
defaults the hour cycle to the locale's preferred hour cycle, if the
best format didn't have an equivalent hour pattern, [[pattern12]] will
be empty.
This adds plumbing for the Intl.DateTimeFormat object, constructor, and
prototype.
Note that unlike other Intl objects, the Intl.DateTimeFormat object has
a LibUnicode structure as a base. This is to prevent wild amounts of
code duplication between LibUnicode, Intl.DateTimeFormat, and other
not-yet-defined Intl structures, because there's 12 fields shared
between them.
