irony-server: do not unnecessarily reparse the file in codeCompleteAt requests

*mind blown*, after all this time I learned by chance that it wasn't
necessary to parse the TU for a clang_codeCompleteAt() request! This
should make the completion visibly faster now.

Discovered here: http://clang-developers.42468.n3.nabble.com/Tips-for-optimising-libclang-td4031257.html