DI: Use Metadata for DITypeRef and DIScopeRef

Now that `MDString` and `MDNode` have a common base class, use it.  Note
that it's not useful to assume subclasses of `Metadata` must be one or
the other since we'll be adding more subclasses soon enough.

Part of PR21532.
This patch fixes couple of bugs for predefined expression
used inside blocks. It fixes a crash in naming code
for __func__ etc. when used in a block declared globally.
It also brings back old naming convention for
predefined expression which was broken. rdar://18961148
DIBuilder: Cleanup access control style, NFC