[VFS] vfs::directory_iterator yields path and file type instead of full Status

Summary:
Most callers I can find are using only `getName()`. Type is used by the
recursive iterator.

Now we don't have to call stat() on every listed file (on most platforms).
Exceptions are e.g. Solaris where readdir() doesn't include type information.
On those platforms we'll still stat() - see D51918.

The result is significantly faster (stat() can be slow).
My motivation: this may allow us to improve clang IO on large TUs with long
include search paths. Caching readdir() results may allow us to skip many stat()
and open() operations on nonexistent files.

Reviewers: bkramer

Subscribers: fedor.sergeev, cfe-commits

Differential Revision: https://reviews.llvm.org/D51921

llvm-svn: 342232