[DebugInfo][RemoveDIs] Add flag to use "new" debug-info in opt (#71937)

Our option to turn on the non-intrinsic form of debug-info
(`--experimental-debuginfo-iterators`) currently requires that LLVM is
built with the `LLVM_EXPERIMENTAL_DEBUGINFO_ITERATORS` cmake flag
enabled, so that some (slight) performance regressions aren't
on-by-default during the prototype/testing period. However, we still
want to be able to _optionally_ run tests, if support is built into
LLVM.

To allow optionally exercising the non-intrinsic debug-info code, this
patch adds `--try-experimental-debuginfo-iterators` to opt, which turns
the `--experimental-debuginfo-iterators` flag on if support is built in,
or leaves it off. This means we can run tests that:
 * Use normal dbg.value intrinsics if there's no support, or
 * Uses non-instruction DPValues if there is support.
  
Which means we can start getting test coverage of DPValues/RemoveDIs
behaviour, from in-tree tests, on our RemoveDIs buildbot. All the code
to do with automagically converting from one form to the other landed in
10a9e744.