In process_override(), stop extracting info we don't use.
Make "bases" and "derived" searches work. Ref bug 1030312.

* Make some changes to the compiler plugin's "impl" handling. Eliminate the loc fields we don't need. Rename the others to be more conventional. Leave the access specifiers for now, though we're not using them.
* process_impl() is now solely a whole-program pass and builds a global class hierarchy. Remove it from the per-file pass.
* Change needles_from_graph() to uniquify its needles. Otherwise, duplicates in the inheritance graph are possible, causing the highlighter to spit out harmless but annoying empty tags.
* Delete a bunch of now-dead code, including group_loc_name, _relate, and build_inheritance.
* SingleFileTestCase.found_lines_eq() no longer requires you to provide line numbers.
* Delete several now-invalid unit tests from test_clang. test_inheritance covers it.
* Swap the names of ParentFilter and ChildFilter, which were arguably wrong.