[llvm-ar] Implement the O modifier: display member offsets inside the archive

Since GNU ar 2.31, the 't' operation prints member offsets beside file
names if the 'O' modifier is specified. 'O' is ignored for thin
archives.

Reviewed By: gbreynoo, ruiu

Differential Revision: https://reviews.llvm.org/D69087

llvm-svn: 375106