Allow long non-file arguments to be spilled

Allow long non-file arguments (e.g. -classpath) to go to a file.  This
prevents E2BIG when the classpath argument is too long.

This can't easily be fixed in upstream ant because:
(a) it would require an API change, and
(b) there is no way for ant to know which arguments are safe to spill

Fixes #671

RELNOTES: N/A

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167726515

ant/src/main/java/com/google/errorprone/ErrorProneExternalCompilerAdapter.java

@@ -98,10 +98,14 @@ public boolean execute() throws BuildException {
       }
 
       cmd.createArgument().setValue(ErrorProneCompiler.class.getName());
+      // This is the first argument that may be spilled to a file.
+      // The ant API describes it as the first argument which is a
+      // file, but in fact only uses it for spilling.  Putting the
+      // break here allows long classpath arguments to be handled.
+      int firstSpillableArgument = cmd.size();
       setupModernJavacCommandlineSwitches(cmd);
-      int firstFile = cmd.size();
       logAndAddFilesToCompile(cmd);
-      return executeExternalCompile(cmd.getCommandline(), firstFile, true) == 0;
+      return executeExternalCompile(cmd.getCommandline(), firstSpillableArgument, true) == 0;
     } else {
       attributes.log(
           "You must set fork=\"yes\" to use the external Error Prone compiler", Project.MSG_ERR);