C++: cache results of linksForIdentifier in HelpItem.

The method linksForIdentifier was called at least thrice when generating
a single valid tooltip in the C++ editor. Now the cached, and the cache
can be "initialised" during construction. This reduces the time spent
for creating a tooltip by 30%.

Task-number: QTCREATORBUG-8970
Change-Id: I5130b769e977c6ffced1a87715831386ef0d5319
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>