llvmPackages_git.libcxx: use clang from git instead of the stdenv's compiler

Port of ca59a201ca1 ("llvmPackages_15.libcxx: use clang 15 instead of
the stdenv's compiler").

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

···
 
     libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;
 
-    libcxx = callPackage ./libcxx {
-      inherit llvm_meta;
-      stdenv = if stdenv.hostPlatform.useLLVM or false
-               then overrideCC stdenv buildLlvmTools.clangNoLibcxx
-               else stdenv;
-    };
-
     libcxxabi = let
-      stdenv_ = if stdenv.hostPlatform.useLLVM or false
-               then overrideCC stdenv buildLlvmTools.clangNoLibcxx
-               else stdenv;
+      # CMake will "require" a compiler capable of compiling C++ programs
+      # cxx-header's build does not actually use one so it doesn't really matter
+      # what stdenv we use here, as long as CMake is happy.
       cxx-headers = callPackage ./libcxx {
         inherit llvm_meta;
-        stdenv = stdenv_;
         headersOnly = true;
       };
+
+      # `libcxxabi` *doesn't* need a compiler with a working C++ stdlib but it
+      # *does* need a relatively modern C++ compiler (see:
+      # https://releases.llvm.org/15.0.0/projects/libcxx/docs/index.html#platform-and-compiler-support).
+      #
+      # So, we use the clang from this LLVM package set, like libc++
+      # "boostrapping builds" do:
+      # https://releases.llvm.org/15.0.0/projects/libcxx/docs/BuildingLibcxx.html#bootstrapping-build
+      #
+      # We cannot use `clangNoLibcxx` because that contains `compiler-rt` which,
+      # on macOS, depends on `libcxxabi`, thus forming a cycle.
+      stdenv_ = overrideCC stdenv buildLlvmTools.clangNoCompilerRtWithLibc;
     in callPackage ./libcxxabi {
       stdenv = stdenv_;
       inherit llvm_meta cxx-headers;
+    };
+
+    # Like `libcxxabi` above, `libcxx` requires a fairly modern C++ compiler,
+    # so: we use the clang from this LLVM package set instead of the regular
+    # stdenv's compiler.
+    libcxx = callPackage ./libcxx {
+      inherit llvm_meta;
+      stdenv = overrideCC stdenv buildLlvmTools.clangNoLibcxx;
     };
 
     libunwind = callPackage ./libunwind {

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

···
   cmakeFlags = [
     "-DLLVM_ENABLE_RUNTIMES=libcxxabi"
     "-DLIBCXXABI_LIBCXX_INCLUDES=${cxx-headers}/include/c++/v1"
+
+    # `libcxxabi`'s build does not need a toolchain with a c++ stdlib attached
+    # (we specify the headers it should use explicitly above).
+    #
+    # CMake however checks for this anyways; this flag tells it not to. See:
+    # https://github.com/llvm/llvm-project/blob/4bd3f3759259548e159aeba5c76efb9a0864e6fa/llvm/runtimes/CMakeLists.txt#L243
+    "-DCMAKE_CXX_COMPILER_WORKS=ON"
   ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"