DebugInfo: Remove DIScope

DebugInfo: Remove DIScope

Replace uses of `DIScope` with `MDScope*`.  There was one spot where
I've left an `MDScope*` uninitialized (where `DIScope` would have been
default-initialized to `nullptr`) -- this is intentional, since the
if/else that follows should unconditional assign it to a value.

llvm-svn: 235327