qdoc: Identify special constructors and assignment ops

This change enables clangqdoc to identify copy constructors,
move-copy constructors, copy-assignment operators and
move-assignment operators. Identifying these special member
functions allows clangqdoc to document them automatically
if the documentation is missing, which also means fewer qdoc
error reports.

Change-Id: Ic50822c2939f0a84e707a1b3ff946bc731a0bd85
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>