[llvm-ar] Make paths case insensitive when on windows

When on windows gnu-ar treats member names as case insensitive. This
commit implements the same behaviour.

Differential Revision: https://reviews.llvm.org/D68033

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375002 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed