Use llvm for dumping DWARF expressions

Summary:
It uses the new ability of ABI plugins to vend llvm::MCRegisterInfo
structs (which is what is needed to turn dwarf register numbers into
strings).

Reviewers: JDevlieghere, aprantl, jasonmolenda

Subscribers: tatyana-krasnukha, lldb-commits

Differential Revision: https://reviews.llvm.org/D67966

llvm-svn: 373208