Add MappedFileRegionArena which can be served as a file system backed
persistent memory allocator. The allocator works like a
BumpPtrAllocator,
and is designed to be thread safe and process safe.

The implementation relies on the POSIX compliance of file system and
doesn't work on all file systems. If the file system supports lazy tail
(doesn't allocate disk space if the tail of the large file is not used),
user has more flexibility to declare a larger capacity.

The allocator works by using a atomically updated bump ptr at a location
that can be customized by the user. The atomic pointer points to the
next available space to allocate, and the allocator will resize/truncate
to current usage once all clients closed the allocator.

Windows implementation contributed by: @hjyamauchi

Add parameter to file lock API to allow exclusive file lock. Both Unix
and Windows support lock the file exclusively for write for one process
and LLVM OnDiskCAS uses exclusive file lock to coordinate CAS creation.

NumRead is already of ssize_t.

This patch removes the C++
disablezOSAutoConversion,enablezOSAutoConversion declarations and also
updates Path.inc to use the common function.

This PR is part of a series to upstream managarm support, as laid out in
the
[RFC](https://discourse.llvm.org/t/rfc-new-proposed-managarm-support-for-llvm-and-clang-87845/85884/1).
This PR is a follow-up to #87845.

The intention here is to add the managarm target to the LLVM support
lib, in preparation for a follow-up that would add the missing clang
driver bits.

Closes #124652.

This header was introduced in
https://github.com/llvm/llvm-project/commit/536736995bf5d073853c7e884968c9847b4ae64d,
but it appears that including only `fnctl.h` should be enough.
Hopefully, this patch will not cause build issues on other Unix
platforms.

It is sufficient to just use `HAVE_DLOPEN`.

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