[DebugInfo][RemoveDIs] Add conversion utilities for new-debug-info format

This patch plumbs the command line --experimental-debuginfo-iterators flag
in to the pass managers, so that modules can be converted to the new
format, passes run, then converted back to the old format. That allows
developers to test-out the new debuginfo representation across some part of
LLVM with no further work, and from the command line. It also installs
flag-catchers at the various points that bitcode and textual IR can egress
from a process, and temporarily convert the module to dbg.value format when
doing so.

No tests alas as it's designed to be transparent.

Differential Revision: https://reviews.llvm.org/D154372