[dsymutil] Add a new automatic verification mode

This patch a new verification mode called "auto" that runs the DWARF
verifier on the input and if the input is valid, also runs the DWARF
verifier on the output. The goal is to catch cases where dsymutil turns
valid DWARF into invalid DWARF. This patch makes this verification mode
the default when assertions or expensive checks are enabled.

Differential revision: https://reviews.llvm.org/D147203