[VFS] Cleanups to VFS interfaces.

Summary:
- add comments clarifying semantics
- Status::copyWithNewName(Status, Name) --> instance method
- Status::copyWithNewName(fs::file_status, Name) --> constructor (it's not a copy)
- File::getName() -> getRealPath(), reflecting its actual behavior/function
  and stop returning status().getName() in the base class (callers can do this
  fallback if they want to, it complicates the contracts).

This is mostly NFC, but the behavior of File::getName() affects FileManager's
FileEntry::tryGetRealPathName(), which now fails in more cases:
 - non-real file cases
 - real-file cases where the underlying vfs::File was opened in a way that
   doesn't call realpath().
(In these cases we don't know a distinct real name, so in principle it seems OK)

Reviewers: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49724

llvm-svn: 337834