kdev-clang: Offer all include paths for code completion

We used to only offer code completion of project paths for local
code completion in #include "..." contexts. And for vice versa,
we only offered system paths for global code completion in
#include <...> contexts. This is wrong, as the include style
only changes the order in which a compiler iterates through these
paths to find an include file. For code completion purposes, this
is not important.

Now we offer code completion in both path lists always. To show the
user the right file/dir being included, we don't sort and unify the
search path list anymore as that potentially changes the final result.
Rather, we use a hash set to ensure we don't encounter paths multiple
times and iterate over the search path lists in their original order.

BUG: 386421