The following scenario is how it breaks:
1. user has ExcelA1 as her formula syntax setting, CONV_UNSPECIFIED
(that means "same as formula syntax") as her string ref syntax setting
2. she saves the document, it will now contain CONV_UNSPECIFIED value
3. someone else with CalcA1 formula syntax setting opens the document
... since it contains CONV_UNSPECIFIED "same as formula syntax" value,
it will use his CalcA1 formula syntax value to evaluate INDIRECT func
=> #REF!

Avoid this by reading formula syntax/grammar value, mapping it to
matching string ref syntax and saving that instead of CONV_UNSPECIFIED

Change-Id: Ide62d81e6b70c2e9f2ff79648935788107475778
Reviewed-on: https://gerrit.libreoffice.org/19610Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>
Reviewed-on: https://gerrit.libreoffice.org/19983Tested-by: Jenkins <ci@libreoffice.org>
(cherry picked from commit c3293e0e)