llvmPackages_11.libcxx: backport block.h fix from llvm12

As a concrete example, this fixes graphviz compilation:

* https://gitlab.com/graphviz/graphviz/-/issues/1785

pkgs/development/compilers/llvm/11/libc++/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetch, cmake, python3, libcxxabi, llvm, fixDarwinDylibNames, version
+{ lib, stdenv, fetch, fetchpatch, cmake, python3, libcxxabi, llvm, fixDarwinDylibNames, version
 , enableShared ? !stdenv.hostPlatform.isStatic
 }:
 
@@ -15,7 +15,14 @@ stdenv.mkDerivation {
     mv llvm-* llvm
   '';
 
-  patches = lib.optional stdenv.hostPlatform.isMusl ../../libcxx-0001-musl-hacks.patch;
+  patches = [
+    (fetchpatch {
+      # Backported from LLVM 12, avoids clashes with commonly used "block.h" header.
+      url = "https://github.com/llvm/llvm-project/commit/19bc9ea480b60b607a3e303f20c7a3a2ea553369.patch";
+      sha256 = "sha256-aWa66ogmPkG0xHzSfcpD0qZyZQcNKwLV44js4eiun78=";
+      stripLen = 1;
+    })
+  ] ++ lib.optional stdenv.hostPlatform.isMusl ../../libcxx-0001-musl-hacks.patch;
 
   preConfigure = lib.optionalString stdenv.hostPlatform.isMusl ''
     patchShebangs utils/cat_files.py