Introduce a java_source_locationt to contain Java specific source location information

[thk123]

Currently the language agnostic contains the Java bytecode index and (as of #1812) the remove_virtual_call flag. These should be pulled into a java_bytecode_source_locationt.

[thomasspriggs]

Yes, separating out a Java specific variation of source_locationt, sounds like it would be a worthwhile refactor. However, given how long this issue has been open it doesn't seem to be a priority for anyone to actually get done. Therefore I am going to close out this issue, rather than leaving it open indefinitely.