DebugInfo: Gut DIVariable and DIGlobalVariable

DebugInfo: Gut DIVariable and DIGlobalVariable

Gut all the non-pointer API from the variable wrappers, except an
implicit conversion from `DIGlobalVariable` to `DIDescriptor`.  Note
that if you're updating out-of-tree code, `DIVariable` wraps
`MDLocalVariable` (`MDVariable` is a common base class shared with
`MDGlobalVariable`).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234840 91177308-0d34-0410-b5e6-96231b3b80d8