SI-6532 emit debug info in compiled java.

Our handful of java source files weren't being compiled
with line numbers, sourcefile, and other debugger aids.
I don't really know how to test this so I'll enclose an
excerpt of the bytecode diff of scala.runtime.IntRef to show
that this change results in debug information.

  2,3c2,4
  >   Compiled from "IntRef.java"
  4a6
  >   SourceFile: "IntRef.java"
  53a62,67
  >       LineNumberTable:
  >         line 18: 0
  >       LocalVariableTable:
  >         Start  Length  Slot  Name   Signature
  >                0      10     0  this   Lscala/runtime/IntRef;
  >                0      10     1  elem   I
  62a77,81
  >       LineNumberTable:
  >         line 19: 0
  >       LocalVariableTable:
  >         Start  Length  Slot  Name   Signature
  >                0       8     0  this   Lscala/runtime/IntRef;
Merge pull request #2471 from paulp/pr/2.10.2/6532

SI-6532 emit debug info in compiled java.