moc: fix Q_INVOKABLE returning references

moc: fix Q_INVOKABLE returning references

The moc generated code would not compile otherwise

Keep Moc::parseFunction and Moc::parseMaybeFunction in sync
(the first is used for signals and slots, and the second for normal
 functions such as Q_INVOKABLE)
Last patch that introduced function pointer updated parseFunction
but not parseMaybeFunction

When a slot return a reference, moc generate code that make the
MetaObject system think it is a void, so qt_metacall and invokeMethod
do not mess with the return value.
But when we want to take the function signature, in the IndexOfMethod
call, we need to have the exact return type.

Change-Id: I4661218d7ce367ad3934e73929e7d04f0a6dbc09
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>