fdo#72041: Intern strings in the external ref cache with the host document.
fdo#72041: Intern strings in the external ref cache with the host document.

To ensure that string comparison with those from the external ref cache
works correctly in functions such as VLOOKUP.

Change-Id: Ib5a466cb6c4b26439abe61b0c17406fc8139f6c0

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core