Backport Fix #1157: Negative number literals should be printed with parentheses.

Backport: No change to javascript printer, since it does not exist in
0.5.x.

(cherry picked from commit acf05682c90a3d543054316eced890698031adef)

Conflicts:

	tools/shared/src/main/scala/scala/scalajs/tools/javascript/Printers.scala