llvmPackages_git.libcxx: updates from LLVM15

None of the patches required any touch-up; the only change of note is:
  - due to changes in the libc++/libc++abi build
    (https://reviews.llvm.org/D120719 and https://reviews.llvm.org/D131037)
    we have to add an extra build option to the libc++ header only
    build that sidesteps bits of the libc++ build config that assume
    libc++-abi is present in the build:
    https://github.com/llvm/llvm-project/blob/4f827318e3e8ccab4ff131e06234caa827e91e4e/libcxx/src/CMakeLists.txt#L255-L256

Rather than maintaining a precise set of build options that let us dodge
referencing libc++-abi variables in the libc++ header only build, we set
`LIBCXX_CXX_ABI` to `none`, as suggested by @lovesegfault.

More discussion about this here: #194634 (comment)

Co-authored-by: Bernardo Meurer <bernardo@meurer.org>

pkgs/development/compilers/llvm/git/libcxx/default.nix

@@ -66,7 +66,7 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [
     "-DLLVM_ENABLE_RUNTIMES=libcxx"
-    "-DLIBCXX_CXX_ABI=${lib.optionalString (!headersOnly) "system-"}${cxxabi.pname}"
+    "-DLIBCXX_CXX_ABI=${if headersOnly then "none" else libcxx_cxx_abi_opt}"
   ] ++ lib.optional (!headersOnly && cxxabi.pname == "libcxxabi") "-DLIBCXX_CXX_ABI_INCLUDE_PATHS=${cxxabi.dev}/include/c++/v1"
     ++ lib.optional (stdenv.hostPlatform.isMusl || stdenv.hostPlatform.isWasi) "-DLIBCXX_HAS_MUSL_LIBC=1"
     ++ lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON"