qdoc: Better support for multi-\fn documentation blocks

This change is a partial fix for the multi-\fn documentation concept.
It outputs the documentation once but listing all the function signatures
from the \fn commands found in the qdoc comment.

Multiple \since commands are not implemented; the \until command is not
implemented, and providing text applicable to a specific \fn signature is
not implemented.

This change requires clang, which means it requires a sequence of other
updates as well, so you can't test it unless you have all that stuff.

Task-number: QTBUG-60420
Change-Id: Ib316b6f97fa427ef730c4badfc785101bff55dce
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>