These FIXMEs were added to keep the dbg_record implementation identical to the
dbg intrinsic versions, which have since been removed. I don't think there's any
reason for the old behaviour; my understanding is it was a minor bug no one got
round to fixing.

I've upgraded the test to be written with dbg_records while I'm here.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

This flag was used to let us incrementally introduce debug records
into LLVM, however everything is now using records. It serves no
purpose now, so delete it.

d81d9e8b8604c85709de0a22bb8dd672a28f0401 changed SplitEdge() to make use
of ehAwareSplitEdge() for critical edges where the target is an eh pad.
However, the implementation is incorrect at least for landing pads. What
is currently produced for the code in the modified unit test is
something like this:

    continue:
      invoke void @sink()
              to label %normal unwind label %new_bb

    new_bb:
      %cp = cleanuppad within %exception []
      cleanupret from %cp unwind label %exception

    exception:
      %cleanup = landingpad i8 cleanup
      br label %trivial-eh-handler

This mixes different exception handling mechanisms in a nonsensical way,
and is not well-formed IR. To actually "split" the landingpad edge (for
a rather loose definition of "split"), I think we'd have to generate
something along these lines:

      exception.split:
        %cleanup.split = landingpad i8 cleanup
        br label %exception.cont

      exception:
        %cleanup.orig = landingpad i8 cleanup
        br label %exception.cont
      
      exception.cont:
%cleanup = phi i8 [ %cleanup.split, %exception_split ], [ %cleanup.orig,
%exception ]

I didn't bother actually implementing that, seeing as how nobody noticed
the existing codegen being broken in the last four years, so clearly
nobody actually needs this function to work with EH edges. Just return
nullptr instead.

DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range.  This patch uses insert_range in
conjunction with llvm::{predecessors,successors} and
MachineBasicBlock::{predecessors,successors}.