llvmPackages_15: updates for LLVM 15

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: NixOS#194634 (comment)

Co-authored-by: Bernardo Meurer <[email protected]>

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

@@ -19,7 +19,7 @@
 }:
 
 let
-  release_version = "14.0.6";
+  release_version = "15.0.6";
   candidate = ""; # empty or "rcN"
   dash-candidate = lib.optionalString (candidate != "") "-${candidate}";
   rev = ""; # When using a Git commit
@@ -31,7 +31,7 @@ let
     owner = "llvm";
     repo = "llvm-project";
     rev = if rev != "" then rev else "llvmorg-${version}";
-    sha256 = "sha256-vffu4HilvYwtzwgq+NlS26m65DGbp6OSSne2aje1yJE=";
+    sha256 = "sha256-Zt/+J6AToJmimlsZPd1huuET6EkNYa6GKgkUFw0Gco8=";
   };
 
   llvm_meta = {
@@ -88,7 +88,7 @@ let
       python3 = pkgs.python3;  # don't use python-boot
     });
 
-    # TODO: lldb/docs/index.rst:155:toctree contains reference to nonexisting document 'design/structureddataplugins'
+    # Needs package for spinhx-automodapi: https://github.com/astropy/sphinx-automodapi
     # lldb-manpages = lowPrio (tools.lldb.override {
     #   enableManpages = true;
     #   python3 = pkgs.python3;  # don't use python-boot

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

@@ -63,7 +63,7 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [
     "-DLLVM_ENABLE_RUNTIMES=libcxx"
-    "-DLIBCXX_CXX_ABI=${lib.optionalString (!headersOnly) "system-"}libcxxabi"
+    "-DLIBCXX_CXX_ABI=${if headersOnly then "none" else "system-libcxxabi"}"
   ] ++ lib.optional (!headersOnly) "-DLIBCXX_CXX_ABI_INCLUDE_PATHS=${libcxxabi.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"