Path: Add an in-place version of path::native.

This reflects the common use case of nativizing a prepared path. The existing
version invokes undefined behavior if input = output, add an assert to catch
that case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190510 91177308-0d34-0410-b5e6-96231b3b80d8