Fix company-rtags completion regex for const member functions
e.g: void Foo::bar(int x) const
Improve performance when rtags updates completions

Previous High CPU Bottlenecks:
- `search-forward-regexp' in `rtags-diagnostics-process-filter'
- `search-forward-regexp' in `rtags-trim-whitespace'
- Copying completion data and parsing as strings instead of using the
original buffer and narrowing
Add hook that runs when completions are received and parsed; provides
rtags-ac with async completion experience

Fix performance bug for rtags-ac that called `rtags-diagnostics' each
time `ac-init' was called (instead of just once)

Don't use a timer to update completions (let completion system initiate)
Merge remote-tracking branch 'remotes/source/master' into completion-improvements
Reset accidental revert of rct submodule to older version
Check last completion location is valid
When completion results arrive, start rtags-ac via `ac-start' to only
show results completion inline; this prevents the case where it auto
expands when only one completion candidate exists, and 'fast' typing
could scramble the symbol name and be generally annoying to the user.
Merge pull request #225 from terranpro/completion-improvements

Completion improvements