Remove duplicated code for synthetic array members.

Summary:
The code for GetSyntheticArrayMemberFromPointer and
GetSyntheticArrayMemberFromArray was identical, so just collapse the
the methods into one.

Reviewers: granata.enrico, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7911

llvm-svn: 230708