CppEditor: fix "follow symbol under cursor" for operators

Like for functions we can call attemptFuncDeclDef for operators. The
check for function names has been enhanced to take operators into
account.
For the switch from definition to declaration
SymbolFinder::findMatchingDeclaration has been enriched with operator
knowledge.

Task-number: QTCREATORBUG-7485

Change-Id: I29eebee337e26c8bf67dc8b4a15c43883045589d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>