Use llvm::sys::path::native instead of llvm::sys::fs::normalize_separators.

llvm::sys::path::native has a superset of the functionality and this was the
only use of llvm::sys::fs::normalize_separators.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215242 91177308-0d34-0410-b5e6-96231b3b80d8