[lldb] Always round down in NSDate's formatter to match NSDate's builtin format

Summary:

When printing an NSDate (for example with `NSLog` or `po`) the seconds value is
always rounded down. LLDB's own formatter however isn't following that behaviour
which leads to situations where the formatted result is sometimes one second
off. For example:

```
(lldb) p [NSDate dateWithTimeIntervalSince1970:0.1]
(__NSTaggedDate *) $1 = [...] 1970-01-01 00:00:01 UTC
(lldb) po [NSDate dateWithTimeIntervalSince1970:0.1]
1970-01-01 00:00:00 +0000

(lldb) p [NSDate dateWithTimeIntervalSince1970:0.6]
(__NSTaggedDate *) $4 =[...] 1970-01-01 00:00:01 UTC
(lldb) po [NSDate dateWithTimeIntervalSince1970:0.6]
1970-01-01 00:00:00 +0000
```

This patch just always rounds down the seconds value we get from the NSDate
object.

Fixes rdar://65084800

Reviewers: mib, davide

Reviewed By: mib

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D83221