Add support for the DWP debug info format

Summary:
The DWP (DWARF package) format is used to pack multiple dwo files
generated by split-dwarf into a single ELF file to make distributing
them easier. It is part of the DWARFv5 spec and can be generated by
dwp or llvm-dwp from a set of dwo files.

Caviats:
* Only the new version of the dwp format is supported (v2 in GNU
  numbering schema and v5 in the DWARF spec). The old version (v1) is
  already deprecated but binutils 2.24 still generates that one.
* Combining DWP files with module debugging is not yet supported.

Subscribers: emaste, mgorny, aprantl

Differential Revision: https://reviews.llvm.org/D36062

llvm-svn: 311775