DebugInfo: Gut DIExpression

Completely gut `DIExpression`, turning it into a simple wrapper around
`MDExpression *`.  There are two bits of magic left:

  - It's constructed from `const MDExpression*` but convertible to
    `MDExpression*`.
  - It's default-constructed to `nullptr`.

Otherwise, it should behave quite like a raw pointer.  Once I've done
the same to the rest of the `DIDescriptor` subclasses, I'll come back to
delete them entirely (and update call sites as necessary to deal with
the missing magic).

llvm-svn: 234832