[lldb][formatters] Add support for printing NSConstantDate and fix distantPast value

Summary:

Certain `NSDate` constructors return a special `NSConstantDate` class which
currently ends up being unformatted as it's not in the list of supported classes
for the NSDate formatter. This patch adds that class to the supported class list
so LLDB produces a summary for it.

One of these special constructors is `[NSDate distantPast]` which returns the
date for `0001-01-01 00:00:00 UTC`. LLDB has a special case for formatting this
date but for some reason we did hardcode the wrong summary string in that
special case. Maybe the summary string was correct back when the code was
written but it isn't correct anymore (`distantPast` isn't actually defined to be
a special date but just some 'a guaranteed temporal boundary.' so maybe someone
changed the value in the last 10 years).

If someone else is wondering why we even have this special case for
`distantPast` but not for the future. The reason seems to be that our date
formatting for really old dates is off by 24 hours. So for example, adding one
second to `distantPast` will cause LLDB to print `0000-12-30 00:00:01 UTC`
(which is 24 hours behind the expected result). So to make our code appear to be
correct it seems we just hardcoded the most common NSDate result from that time
span. I'll replace that logic with a generic solution in a probably more
invasive follow up patch.

I also took the freedom to replace the magic value `-63114076800` with some
constant + documentation. I heard there are some people that don't know from the
top of their head that there are 63114076800 seconds between 1. Jan 0001 and 1.
January 2001 in whatever calendar system NSDate is using.

Reviewers: mib, davide

Reviewed By: mib

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D83217