...to svx::a11y::AccFrameSelector (which are unrelated final classes).  UBSan
Calc "Format - Cells... - Borders - Line Arrangement - User-defined:" and
clicking into the top-right corner of the widget caused

> svx/source/dialog/frmsel.cxx:972:50: runtime error: downcast of address 0x60f0003dfa80 which does not point to an object of type 'a11y::AccFrameSelector'
> 0x60f0003dfa80: note: object is of type 'svx::a11y::AccFrameSelectorChild'
>  e1 03 80 0d  90 4f c2 88 ce 7f 00 00  02 00 00 00 be be be be  00 00 00 00 00 00 00 00  00 00 00 00
>               ^~~~~~~~~~~~~~~~~~~~~~~
>               vptr for 'svx::a11y::AccFrameSelectorChild'
>  #0 in svx::FrameSelector::SelectBorder(svx::FrameBorderType) at svx/source/dialog/frmsel.cxx:972:50
>  #1 in svx::FrameSelector::MouseButtonDown(MouseEvent const&) at svx/source/dialog/frmsel.cxx:1114:21
[...]

The code was like that ever since 60f11adb
"Integrate branch of IAccessible2", which had presumably confused
AccFrameSelector and AccFrameSelectorChild here (while there'd apparently not
been a deeper need for the static_casts anyway).

Change-Id: I1f52629143dcfc84e57e9539a428fb0f51fdc72e
Reviewed-on: https://gerrit.libreoffice.org/72910
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>