Identified with modernize-use-equals-default.

This reverts commit 3b5842c9c41a441280100045ef62bb8a0fe7200f.

The intent of the original commit was to begin enabling asynchronous
alloation actions (calls attached to JIT'd memory initialization and
deinitialization). The asynchronous allocation actions scheme was
fleshed-out in a development branch, but ran into an issue: Functions
implementing actions are allowed to live in JIT'd code (e.g. in the ORC
runtime), but we can't genally rely on tail-call elimination kicking in.
This resulting in dealloc actions returning via stack frames that had
been deallocated, triggering segfaults.

It's possible that there are other approaches that would allow
asynchronous allocation actions to work, but they're not on the critical
path for JIT improvements so for now we'll just revert.

This gives MemoryMapper implementations a chance to allocate working
memory using the LinkGraph's allocator.

Part of ongoing work to make core ORC operations asynchronous.

This only affects the runner utilities, not the AllocationActions themselves.
The AllocationActions will be made asynchronous in a future patch.

This PR will solve the issue with leaking shared memory we have after running llvm lit test on z/OS.
In particular llvm/unittests/ExecutionEngine/Orc/SharedMemoryMapperTest.cpp was causing the leak.

Identified with misc-include-cleaner.

Without these explicit includes, removing other headers, who implicitly
include llvm-config.h, may have non-trivial side effects.

Fix 'use of undeclared identifier' build errors for shm_ functions on
z/OS by implementing the functionality using shmget(), shmat(), and
shmdt(). Use the BLAKE3 hash to map the name of the shared memory to a
key.

---------

Co-authored-by: Kai Nacke <kai.peter.nacke@ibm.com>

LLVM is inconsistent about how it converts `errno` to `std::error_code`.
This can cause problems because values outside of `std::errc` compare
differently if one is system and one is generic on POSIX systems.

This is even more of a problem on Windows where use of the system
category is just wrong, as that is for Windows errors, which have a
completely different mapping than POSIX/generic errors. This patch fixes
one instance of this mistake in `JSONTransport.cpp`.

This patch adds `errnoAsErrorCode()` which makes it so people do not
need to think about this issue in the future. It also cleans up a lot of
usage of `errno` in LLVM and Clang.