Add llvm::sys::path::canonical(), which provides the canonicalized

Add llvm::sys::path::canonical(), which provides the canonicalized
name of a path, after resolving symbolic links and eliminating excess
path elements such as "foo/../" and "./".

This routine still needs a Windows implementation, but I don't have a
Windows machine available. Help? Please?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125228 91177308-0d34-0410-b5e6-96231b3b80d8