Modify ScFormulaCell's ctor to take a const reference to ScTokenArray.

Instead of a pointer to ScTokenArray, and then clone its instance.  If
the token array gets cloned in ctor, take a const reference.

Change-Id: I280fd7eb9eaea9905dbf954a1ace904ab0814dfe
Avoid duplication of ScTokenArray during formula cell construction.

For slightly less overhead.

Change-Id: Ie5861d585d6e22fbd19dfd57edfebae4f4504839