[llvm-ar] Accept file paths with windows format slashes

The internal representation of llvm-ar archives uses linux style slashes
for paths, no matter the OS. In the case of windows this meant file
paths input intending to match existing members would only match if
linux style slashes where used. This change allows either slash
direction to be input by the user.

This change includes removing an unnecessary call to normalisePath and
moving the call of another.

Differential Revision: https://reviews.llvm.org/D65743

llvm-svn: 368573