[FileSpec] Delegate common operations to llvm::sys::path

With the recent changes in FileSpec to use LLVM's path style, it is
possible to delegate a bunch of common path operations to LLVM's path
helpers. This means we only have to maintain a single implementation and
at the same time can benefit from the efforts made by the rest of the
LLVM community.

This is part one of a set of patches. There was no obvious way to split
this so I just worked from top to bottom.

Differential revision: https://reviews.llvm.org/D48084

llvm-svn: 334615