antlr3 generated target has unstable hash and uncache-able due to timestamp in generated code

[qma]

antlr3 puts a comment with timestamp at the top of generated java sources, causing the target artifact to have an ever-changing fp which becomes un-cacheable. Any targets dependent on an antlr target will also be uncacheable due to ever-changing dependent hashes, causing build slowdown. Proposed fix is either

1. scrub the generated sources to remove timestamps
2. change fingerprint strategy for java_antlr_library so that it hashes the original .g source rather than the generated

[jsirois]

2 already happens for the inputs to AntlrGen, its outputs though are the inputs to the JavaCompile task and so that's where the fingerprinting instability comes in. So the fix will have to be #1 in the short to medium term.

[qma]

Implemented #1 in #1701

[stuhood]

Fixed.