ci: Use tsan-instrumented libcxx in sanitizers job

mptest thread sanitizer checks that were failing in bitcoin core CI were not
failing in libmultiprocess CI, and this change instruments libcxx so the same
checks should happen both places.

Credit to MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz> for pointing this out
bitcoin/bitcoin#33518 (comment)

Author: ryanofsky

ci/configs/sanitize.bash

@@ -1,5 +1,6 @@
 CI_DESC="CI job running ThreadSanitizer"
 CI_DIR=build-sanitize
+NIX_ARGS=(--arg enableLibcxx true --argstr libcxxSanitizers "Thread")
 export CXX=clang++
 export CXXFLAGS="-ggdb -Werror -Wall -Wextra -Wpedantic -Wthread-safety -Wno-unused-parameter -fsanitize=thread"
 CMAKE_ARGS=()

shell.nix

@@ -4,11 +4,17 @@
 , minimal ? false # Whether to create minimal shell without extra tools (faster when cross compiling)
 , capnprotoVersion ? null
 , cmakeVersion ? null
+, libcxxSanitizers ? null # Optional LLVM_USE_SANITIZER value to use for libc++, see https://llvm.org/docs/CMake.html
 }:
 
 let
   lib  = pkgs.lib;
-  llvm = crossPkgs.llvmPackages_20;
+  llvmBase = crossPkgs.llvmPackages_21;
+  llvm = llvmBase // lib.optionalAttrs (libcxxSanitizers != null) {
+    libcxx = llvmBase.libcxx.override {
+      devExtraCmakeFlags = [ "-DLLVM_USE_SANITIZER=${libcxxSanitizers}" ];
+    };
+  };
   capnprotoHashes = {
     "0.7.0" = "sha256-Y/7dUOQPDHjniuKNRw3j8dG1NI9f/aRWpf8V0WzV9k8=";
     "0.7.1" = "sha256-3cBpVmpvCXyqPUXDp12vCFCk32ZXWpkdOliNH37UwWE=";