[dsymutil] Add preliminary support for DWARF 5.
Currently dsymutil will silently fail when processing binaries with
Dwarf 5 debug info. This patch adds rudimentary support for Dwarf 5 in
- Recognize relocations in the debug_addr section.
- Recognize (a subset of) Dwarf 5 form values.
- Emits valid Dwarf 5 compile unit header chains.
To simplify things (and avoid having to emit indexed sections) I decided
to emit the relocated addresses directly in the debug info section.
- DW_FORM_strx gets relocated and rewritten to DW_FORM_strp
- DW_FORM_addrx gets relocated and rewritten to DW_FORM_addr
Obviously there's a lot of work left, but this should be a step in the
Differential revision: https://reviews.llvm.org/D94323