FREQUENCY() forces its direct caller into array mode, but only for
the immediate subexpression and not for further operators of the
same parameter.

This weird Excel behaviour is stated in

ECMA-376-1:2016 OOXML 18.17.7.127 FREQUENCY
"A call to FREQUENCY shall be an array formula."

somewhat unclear what it actually applies to, but it turned out
that "a call" is indeed *only* THE direct call, see
https://bugs.documentfoundation.org/show_bug.cgi?id=122301#c19

Change-Id: I145d8fe26d75d5af25b987e190bf35f2d2c03ec6
Reviewed-on: https://gerrit.libreoffice.org/66407Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins