[NFC] Add DebugVariable constructor that takes DbgVariableIntrinsic pointer

Note: The constructor definition cannot be inline without some refactoring as
it introduces a circular dependency between the headers
llvm/IR/DebugInfoMetadata.h (this file) and llvm/IR/IntrinsicInst.h (where
DbgVariableIntrinsic is defined).

Reviewed By: jryans

Differential Revision: https://reviews.llvm.org/D133286