Remove Path::isObjectFile.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184305 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the 'R' modifier.

It is not present in GNU or OS X versions and doesn't make a lot of sense
for llvm-ar.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184306 91177308-0d34-0410-b5e6-96231b3b80d8