Remove dependency on old tree along with some fixes in completion list

Changes:
* Remove dependencies on old tree for figuring out the location
* General clean up
* Disallowed completion in interface member name definition sites
* Ensured we walk the tree once to find the token instead of multiple times later on
* Bug fixes:
    * Completion list showing private members of non-containg class #913
    * Crash in completion after a slash #903
    * Completion entry details in new target do not show constructor signature #866
    * Completion List on a line immediately after ambient function shows its parameters as completion list entry #764