[Obj-C] Fix undefined behaviour(s) in the new NSTaggedDate formatter.

Type punning through a union -> no good.
double to uint64 to double again -> no good either.

The nice side effect, other than silencing the sanitizer bot
is that it fixes the formatting of some dates, e.g. Jan 1st 1970.

<rdar://problem/47617983>

llvm-svn: 353191