[DebugInfo][RemoveDIs] Extend intrinsic-conversion in debugify (#80861)

A while back the entry/exit points of debugify were instrumented with
conversion functions to/from non-intrinsic-form debug-info. This is the
path of least resistance to incrementally converting parts of LLVM to
use the new format. However, it turns out that debugify registers
callbacks with the pass manager and can be fed non-intrinsic form
debug-info. Thus: this patch wraps each of the four major debugify
functions with the convertion utilities, and extends test coverage to a
test that exposes this problem.

(An alternative would be to put this code in the callback lambdas, but
then it would be fighting pass manager abstractions of what type the IR
has).

Handily debugify has been designed to record the /meaning/ of debug-info
rather than take pointers to intrinsics and the like, so the storage
mechanism for debug-info is transparent to it!