[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
dsymutil.

 - 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
right direction.

rdar://62345491

Differential revision: https://reviews.llvm.org/D94323