Add missing implementation of 'sys::path::is_other' to the support library.

The header claims that this function exists, but the linker wasn't too happy
about it not being in the library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224527 91177308-0d34-0410-b5e6-96231b3b80d8