[lldb][NFCI] Switch to using llvm::DWARFAbbreviationDeclaration

Both LLVM and LLDB implement DWARFAbbreviationDeclaration. As of
631ff46c, llvm's implementation of
DWARFAbbreviationDeclaration::extract behaves the same as LLDB's
implementation, making it easier to merge the implementations.

The only major difference between LLDB's implementation and LLVM's
implementation is that LLVM's DWARFAbbreviationDeclaration is slightly
larger. Specifically, it has some metadata that keeps track of the size
of a declaration (if it has a fixed size) so that it can potentially
optimize extraction in some scenarios. I think this increase in size
should be acceptable and possibly useful on the LLDB side.

Differential Revision: https://reviews.llvm.org/D150716