qdoc no longer creates a function node when it parses a function
declaration, if a function node for the function already exists.
This avoids needless allocation and deallocation.

qdoc now also avoids creating unnecessary variable nodes.

This change also improves the "No documentation for..." error
message so that it includes the parameters if the error refers
to a function. This makes it easier to find the correct function
when you are trying to correct these errors in the documentation.

Change-Id: Ib0f9361622f9115d677408e36bfed77c42545be0
Task-number: QTBUG-48191
Reviewed-by: Martin Smith <martin.smith@digia.com>

we can do that now, as the bootstrap lib is now a properly exported
module, and qmldevtools is now bootstrapped as well.

this removes the abomination of a copy of the qml parser in qtbase.
unfortunately qtbase/2422251ee5025a067b14b989153764ab36e43f10 is
reverted, as qtdeclarative is still missing the respective change.

this introduces no regression in discoverability or usability, as a full
doc build already needed qttools - for qhelpgenerator.

Change-Id: Ic9c4c9732ddf5998637b9e42e27939ba50b31479
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>