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