[libc++] Use a different namespace for the debug-mode version of the library

[ldionne]

Summary

This ensures that one can't easily mix the debug version of the library
and the non-debug version of the library by mistake.

[philnik]

libcxx/CMakeLists.txt

191

I think versioning the debug namespace in the same way we version the non-debug namespace isn't a good idea. It gives the impression that the ABI for the debug mode is stable. I would suggest just using __libcxx_debug_ns as the namespace or use the libc++ version in the mangling to catch these kinds of mismatches as well. Maybe __libcxx_debug_<version>?

[Mordante]

libcxx/CMakeLists.txt

191

But I assume we can still use ABI v1 and ABI v2 with debug mode. In which case this needs to remain.

[ldionne]

Abandoning since the legacy debug mode doesn't exist anymore.

libcxx/CMakeLists.txt

 set(LIBCXX_ABI_VERSION "${abi_version}" CACHE STRING
   "ABI version of libc++. Can be either 1 or 2, where 2 is currently the unstable ABI.
    Defaults to 1 unless LIBCXX_ABI_UNSTABLE is specified, in which case this is 2.")
 set(LIBCXX_LIBRARY_VERSION "${LIBCXX_ABI_VERSION}.0" CACHE STRING
   "Version of libc++. This will be reflected in the name of the shared library produced.
    For example, -DLIBCXX_LIBRARY_VERSION=x.y will result in the library being named
    libc++.x.y.dylib, along with the usual symlinks pointing to that. On Apple platforms,
    this also controls the linker's 'current_version' property.")
-set(LIBCXX_ABI_NAMESPACE "__${LIBCXX_ABI_VERSION}" CACHE STRING "The inline ABI namespace used by libc++. It defaults to __n where `n` is the current ABI version.")
+
+set(_default_abi_namespace "__${LIBCXX_ABI_VERSION}")
+if (LIBCXX_ENABLE_DEBUG_MODE)
+  set(_default_abi_namespace "${_default_abi_namespace}d")
+endif()
+set(LIBCXX_ABI_NAMESPACE "${_default_abi_namespace}" CACHE STRING
+  "The inline ABI namespace used by libc++. It defaults to __n where `n` is the current ABI version.
+   If the debug mode is enabled, the 'd' suffix is added to the namespace to distinguish it from the
+   non-debug version of the library.")
 if (NOT LIBCXX_ABI_NAMESPACE MATCHES "__.*")
   message(FATAL_ERROR "LIBCXX_ABI_NAMESPACE must be a reserved identifier.")
 endif()
 option(LIBCXX_ABI_FORCE_ITANIUM "Ignore auto-detection and force use of the Itanium ABI.")
 option(LIBCXX_ABI_FORCE_MICROSOFT "Ignore auto-detection and force use of the Microsoft ABI.")
 
 set(LIBCXX_TYPEINFO_COMPARISON_IMPLEMENTATION "default" CACHE STRING
   "Override the implementation to use for comparing typeinfos. By default, this