### What changes were proposed in this pull request?
Make separate source trees for Scala 2.12/2.13 in order to accommodate mutually-incompatible support for Ordering of double, float.
Note: This isn't the last change that will need a split source tree for 2.13. But this particular change could go several ways:
- (Split source tree)
- Inline the Scala 2.12 implementation
- Reflection
For this change alone any are possible, and splitting the source tree is a bit overkill. But if it will be necessary for other JIRAs (see umbrella SPARK-25075), then it might be the easiest way to implement this.
### Why are the changes needed?
Scala 2.13 split Ordering.Double into Ordering.Double.TotalOrdering and Ordering.Double.IeeeOrdering. Neither can be used in a single build that supports 2.12 and 2.13.
TotalOrdering works like java.lang.Double.compare. IeeeOrdering works like Scala 2.12 Ordering.Double. They differ in how NaN is handled - compares always above other values? or always compares as 'false'? In theory they have different uses: TotalOrdering is important if floating-point values are sorted. IeeeOrdering behaves like 2.12 and JVM comparison operators.
I chose TotalOrdering as I think we care more about stable sorting, and because elsewhere we rely on java.lang comparisons. It is also possible to support with two methods.
### Does this PR introduce any user-facing change?
Pending tests, will see if it obviously affects any sort order. We need to see if it changes NaN sort order.
### How was this patch tested?
Existing tests so far.
Closes#26654 from srowen/SPARK-30009.
Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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