[FileSystem] Move EnumerateDirectory from FileSpec to FileSystem.

This patch moves the EnumerateDirectory functionality and related enum
and typedef from FileSpec to FileSystem.

This is part of a set of patches that extracts file system related
convenience methods from FileSpec. The long term goal is to remove this
method altogether and use the iterators directly, but for introducing
the VFS into LLDB this change is sufficient.

Differential revision: https://reviews.llvm.org/D53785

llvm-svn: 345800