[llvm-ar] Prevent automatic conversion from thin to full archive

llvm-ar silently converts a thin archive to a regular archive when you
specify a modification operation (e.g. 'r') without the 'T' modifier.
This change stops this from occuring. If a user is trying to convert
between thin and full archives then they can explicitly use the 'L'
command to createa new archive.

Differential Revision: https://reviews.llvm.org/D118693