[llvm-dwp] Initial partial prototype

[llvm-dwp] Initial partial prototype

This just concatenates the common DWP sections without doing any of the
fancy DWP things like:

1) update str_offsets
2) deduplicating strings
3) merging/creating cu/tu_index

Patches for these will follow shortly.

(also not sure about target triple/object file type for this tool - do I
really need a whole triple just to write an object file that contains
purely static/hardcoded bytes in each section? & I guess I should just
pick it based on the first input, maybe, rather than hardcoding for now
- but we only produce .dwo on ELF platforms with objcopy for now anyway)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254355 91177308-0d34-0410-b5e6-96231b3b80d8