test case is exporting ooo55266-3 (contains data grouped in numerical
intervals) to xlsx and without closing the document, opening filter on
1st pivot table (kaboom!)

ClearGroupFields corrupts the cache bc it resets Field.mpGroup items
for all dimensions, not just the one present in ScDPDimensionSaveData
(all this happening in ScDPCollection::SheetCaches::getCache).
Consequently, accessing or rebuilding pivot cache may crash bc mpGroup
now points nowhere.

I split and renamed ScDPCache::ClearGroupFields into 2 parts, one of
them clears maGroupFields, the other resets mpGroup ptrs in maFields.
When adding data to cache, the former is used (bc group ptrs get reset
almost immediately afterwards)

Change-Id: I96e8d234a17da0f3cc65c0625aa47b12284b98b8
Reviewed-on: https://gerrit.libreoffice.org/65329
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>