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.

This was a utility for flipping between intrinsic and debug record mode
-- we don't need it any more. The "IsNewDbgInfoFormat" should be true
everywhere.

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.

The type mapping in IRMover currently has a decent amount of complexity
related to establishing isomorphism between opaque struct types and
non-opaque struct types. I believe that this is both largely useless at
this point (after some recent clarifications, essentially the only place
where opaque types can still appear are external gobals) and has never
been entirely correct in the first place (because it does this in part
purely based on name rather than use, which means that we effectively
end up assigning semantics to the type name, which is illegal).

As such, I'd like to remove this functionality entirely.

Consider opaque types as non-first-class types, i.e. do not allow SSA
values to have opaque type.

By calling this function after every link, we introduce quadratic
behavior during full LTO links that primarily affects links involving
large numbers of constant arrays, such as the full LTO part of a
ThinLTO link which will involve large numbers of vtable constants.
Removing this call resulted in a 1.3x speedup in the indexing phase
of a large internal Google binary.

This call was originally introduced to reduce memory consumption during
full LTO (see commit dab999d54f39af3ebb6a23e850dbc9742016fac8), but it
doesn't seem to be the case that this helps any more. I ran 3 stage2
links of clang with full LTO and ThinLTO with and without this change
and measured the max RSS. The results were as follows (all in KB):

```
FullLTO before 22362512 22383524 22387456
         after 22383496 22365356 22364352
ThinLTO before  4391404  4478192  4383468
         after  4399220  4363100  4417688
```

As you can see, any max RSS differences are in the noise.

Reviewers: nikic, teresajohnson

Reviewed By: teresajohnson, nikic

Pull Request: https://github.com/llvm/llvm-project/pull/137081

LLVM no longer supports recursive types, so the Visited set is
unnecessary.