qdoc: Improve handling of errors during \fn signature parsing

Recent changes to QDoc (in dev) caused multiple superfluous warnings to be generated. These are of type

qt5/qtbase/src/network/access/qhttp2connection.cpp:46: (qdoc) warning: clang couldn't find function when parsing \fn quint32 QHttp2Stream::streamID() const noexcept

These are for \fn documentation blocks that are members of \class or \namespace entries that are marked \internal. Therefore, they do not affect the validity of the output, but still have an impact for testing doc generation in CI where we enforce no warnings.

The reason is that the order of parsing topic- and meta-commands have changed, and therefore the Clang code parser has no longer information available about meta-commands (specifically, \internal) assigned to the base class when parsing an \fn.

These errors must be stored for later processing, so that a subclass of them can be safely omitted in above cases.