[clang] [libcxx] [Clang] Implement CWG2369 "Ordering between constraints and substitution" (PR #102857)

[Daniel Thornburgh via cfe-commits]

mysterymath wrote:

We've started seeing libc++ test failures in the Fuchsia toolchain buildbots for Windows: https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-windows-x64/b8726615170786362641/overview

This may be related to the discussion in #121881, but the error looks slightly different.

This is definitely the only plausible PR in the blamelist: https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-windows-x64/b8726615170786362641/blamelist
```
llvm-libc++-static-clangcl.cfg.in :: std/utilities/optional/optional.relops/less_than.pass.cpp
==============
# | In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\libcxx\test\std\utilities\optional\optional.relops\less_than.pass.cpp:14:
# | C:/b/s/w/ir/x/w/llvm_build/runtimes/runtimes-x86_64-pc-windows-msvc-bins/libcxx/test-suite-install/include/c++/v1\optional:1247:1: error: cannot mangle this dependent name type yet
# |  1247 | _LIBCPP_HIDE_FROM_ABI constexpr compare_three_way_result_t<_Tp, _Up>
# |       | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |  1248 | operator<=>(const optional<_Tp>& __x, const _Up& __v) {
# |       | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |  1249 |   return __x.has_value() ? *__x <=> __v : strong_ordering::less;
# |       |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |  1250 | }
# |       | ~
```

https://github.com/llvm/llvm-project/pull/102857