Rename functionDeclarationForCalledDeclaration -> functionForCalled and tweak types.

Take a Declaration* instead of DeclarationPointer.

Returns struct { FunctionDeclaration* declaration, bool isConstructor }
instead of QPair<FunctionDeclarationPointer, bool>.

Struct for readable attribute names rather than .first and .second.

Drop the magic pointer type because most callers only wrap a normal one
for this call anyway.

No test changes, no (intended) behaviour changes.

Differential Revision: https://phabricator.kde.org/D3534