Frontend: Remove CompilerInstance::VirtualFileSystem, NFC

Remove CompilerInstance::VirtualFileSystem and
CompilerInstance::setVirtualFileSystem, instead relying on the VFS in
the FileManager.  CompilerInstance and its clients already went to some
trouble to make these match.  Now they are guaranteed to match.

As part of this, I added a VFS parameter (defaults to nullptr) to
CompilerInstance::createFileManager, to avoid repeating construction
logic in clients that just wanted to customize the VFS.

https://reviews.llvm.org/D59377

llvm-svn: 357037