firebase-ios-sdk

Prep for FIRTimestampValue migration to C++ (#3155)

* Clean up GeoPoint port to C++

... based on things learned porting Timestamps.

There's no good way to include conversions equivalent to toGeoPoint in
FIRTimestamp, so pull out separate conversion functions that match other
similar things.

* Extract MakeTimePoint into time_testing.h

... for reuse elsewhere

* Add TimestampInternal Hash/Truncate

* Use a method similar to NSDate to get current time on apple platforms