llvmPackages_15: copy fix from llvmPackages_16 6ba1b5b

from issue 214524

> I am pretty sure that this is caused by lib/libc++.so in llvmPackages_15.libcxx containing INPUT(libc++.so.1 -l) instead of INPUT(libc++.so.1 -lc++abi) (which it does for llvmPackages_14.libcxx). I don't know yet, what the root cause for that is or how to fix that.

pkgs/development/compilers/llvm/15/default.nix

@@ -249,6 +249,14 @@ in let
         [ "-rtlib=compiler-rt"
           "-Wno-unused-command-line-argument"
           "-B${targetLlvmLibraries.compiler-rt}/lib"
+
+          # Combat "__cxxabi_config.h not found". Maybe this could be fixed by
+          # copying these headers into libcxx? Note that building libcxx
+          # outside of monorepo isn't supported anymore, might be related to
+          # https://github.com/llvm/llvm-project/issues/55632
+          # ("16.0.3 libcxx, libcxxabi: circular build dependencies")
+          # Looks like the machinery changed in https://reviews.llvm.org/D120727.
+          "-I${lib.getDev targetLlvmLibraries.libcxx.cxxabi}/include/c++/v1"
         ]
         ++ lib.optional (!stdenv.targetPlatform.isWasm) "--unwindlib=libunwind"
         ++ lib.optional