These are unneeded even on AIX, PURE_WINDOWS, and ZOS (per #104706)

* HAVE_ERRNO_H: introduced by 1a93330ffa2ae2aa0b49461f05e6f0d51e8443f8 (2009) but unneeded.
  The guarded ABI is unconditionally used by lldb.
* HAVE_FCNTL_H
* HAVE_FENV_H
* HAVE_SYS_STAT_H

Pull Request: https://github.com/llvm/llvm-project/pull/123087

- Fix `HANDLE_EXTENSION` macro redefinition warning in LTOBackend.cpp
- Fix "unnecessary brackets" around rf/df variable definitions warning.

Reviewed By: sgraenitz

Differential Revision: https://reviews.llvm.org/D157226

This reverts commit d941d597837d9e1405086f008c9bd6a71e7263c9.

Differential Revision: https://reviews.llvm.org/D122258

This reverts commit ef501bf85d8c869248e51371f0e74bcec0e7b229.

Previously, omitting unnecessary DWARF unwinds was only done in two
cases:
* For Darwin + aarch64, if no DWARF unwind info is needed for all the
  functions in a TU, then the `__eh_frame` section would be omitted
  entirely. If any one function needed DWARF unwind, then MC would emit
  DWARF unwind entries for all the functions in the TU.
* For watchOS, MC would omit DWARF unwind on a per-function basis, as
  long as compact unwind was available for that function.

This diff makes it so that we omit DWARF unwind on a per-function basis
for Darwin + aarch64 as well. In addition, we introduce the flag
`--emit-dwarf-unwind=` which can toggle between `always`,
`no-compact-unwind` (only emit DWARF when CU cannot be emitted for a
given function), and the target platform `default`.  `no-compact-unwind`
is particularly useful for newer x86_64 platforms: we don't want to omit
DWARF unwind for x86_64 in general due to possible backwards compat
issues, but we should make it possible for people to opt into this
behavior if they are only targeting newer platforms.

**Motivation:** I'm working on adding support for `__eh_frame` to LLD,
but I'm concerned that we would suffer a perf hit. Processing compact
unwind is already expensive, and that's a simpler format than EH frames.
Given that MC currently produces one EH frame entry for every compact
unwind entry, I don't think processing them will be cheap. I tried to do
something clever on LLD's end to drop the unnecessary EH frames at parse
time, but this made the code significantly more complex. So I'm looking
at fixing this at the MC level instead.

**Addendum:** It turns out that there was a latent bug in the X86
backend when `OmitDwarfIfHaveCompactUnwind` is naively enabled, which is
not too surprising given that this combination has not been heretofore
used.

For functions that have unwind info that cannot be encoded with CU, MC
would end up dropping both the compact unwind entry (OK; existing
behavior) as well as the DWARF entries (not OK).  This diff fixes things
so that we emit the DWARF entry, as well as a CU entry with encoding
`UNWIND_X86_MODE_DWARF` -- this basically tells the unwinder to look for
the DWARF entry. I'm not 100% sure the `UNWIND_X86_MODE_DWARF` CU entry
is necessary, this was the simplest fix. ld64 seems to be able to handle
both the absence and presence of this CU entry. Ultimately ld64 (and
LLD) will synthesize `UNWIND_X86_MODE_DWARF` if it is absent, so there
is no impact to the final binary size.

Reviewed By: davide, lhames

Differential Revision: https://reviews.llvm.org/D122258

As described on D111049, we're trying to remove the <string> dependency from error handling and replace uses of report_fatal_error(const std::string&) with the Twine() variant which can be forward declared.

This was already the case, but the recent change (957334382cd1) altered
the behavior on some of our bots where __unw_add_dynamic_fde is not
found. This restores the prior behavior on Darwin while also retaining
the new behavior from that change.

libgcc and libunwind have different flavours of __register_frame. Both
 flavours are already correctly handled, except that the code to handle
the libunwind flavour is guarded by __APPLE__. This change uses the
presence of __unw_add_dynamic_fde in libunwind instead to detect whether
libunwind is used, rather than hardcoding it as Apple vs. non-Apple.

Fixes PR44074.

Thanks to Albert Jin <albert.jin@gmail.com> and Chris Schafmeister
<chris.schaf@verizon.net> for identifying the problem.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D106129