[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