database/sql: scan into *time.Time without reflection

Previously scanning time.Time into a *time.Time required reflection.
Now it does not. Scanning already checked if the source value was of
type time.Time. The only addition was checking the destination was
of type *time.Time.

Existing tests already scan time.Time into *time.Time, so no new
tests were added. Linked issue has performance justification.

Fixes #22300

Change-Id: I4eea461c78fad71ce76e7677c8503a1919666931
Reviewed-on: https://go-review.googlesource.com/73232
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/convert.go

@@ -259,6 +259,9 @@ func convertAssign(dest, src interface{}) error {
 		}
 	case time.Time:
 		switch d := dest.(type) {
+		case *time.Time:
+			*d = s
+			return nil
 		case *string:
 			*d = s.Format(time.RFC3339Nano)
 			return nil