Unfortunately gcc 12's is not yet capable of compiling all of libc++
19's C++23 code, which results in errors similar to:

  /usr/src/freebsd/src/contrib/llvm-project/libcxx/include/__algorithm/ranges_contains.h:41:3: error: 'static constexpr bool std::__1::ranges::__contains::__fn::operator()(_Iter, _Sent, const _Type&, _Proj)' must be a non-static member function
     41 |   operator()(_Iter __first, _Sent __last, const _Type& __value, _Proj __proj = {}) {
        |   ^~~~~~~~
  /usr/src/freebsd/src/contrib/llvm-project/libcxx/include/__algorithm/ranges_contains.h:48:3: error: 'static constexpr bool std::__1::ranges::__contains::__fn::operator()(_Range&&, const _Type&, _Proj)' must be a non-static member function
     48 |   operator()(_Range&& __range, const _Type& __value, _Proj __proj = {}) {
        |   ^~~~~~~~

Until we can get rid of gcc 12, work around this by making it compile
libc++ in C++20 mode instead.

NOTE: The resulting libc++ library will not be C++23 compatible! Please
try to avoid shipping it, and use gcc 13 instead, if you must use gcc.

PR:		280562
MFC after:	3 days

PR:		280562
MFC after:	1 month

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project release/19.x llvmorg-19.1.0-rc1-0-ga4902a36d5c2.

PR:		280562
MFC after:	1 month

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-19-init-18630-gf2ccf80136a0, the
last commit before the upstream release/19.x branch was created.

PR:		280562
MFC after:	1 month

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-18-init-18359-g93248729cfae, the
last commit before the upstream release/18.x branch was created.

PR:		276104
MFC after:	1 month

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-18-init-16864-g3b3ee1f53424.

PR:		276104
MFC after:	1 month

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-18-init-15692-g007ed0dccd6a.

PR:		276104
MFC after:	1 month

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-18-init-15088-gd14ee76181fb.

PR:		276104
MFC after:	1 month

I missed this header while updating the Makefile for libc++ 17.

PR:		273753
MFC after:	1 month