intel-oneapi-toolkit: fix C++ standard library headers in stdenv

The wrapped icpx could not find libstdc++ headers, so any C++ source
failed with e.g. "'type_traits' file not found". This broke the
headers-available and sycl-compile passthru tests.

cc-wrapper only adds libstdc++ include paths when the compiler sets
langCC, which the unwrapped icpx shim did not. Set it.

Assisted-by: Claude Code with Opus 5.5 medium (Anthropic)

pkgs/by-name/in/intel-oneapi-toolkit/stdenv.nix

@@ -30,7 +30,12 @@ let
       chmod +x $out/bin/clang
     '';
 
-    passthru.isClang = true;
+    passthru = {
+      isClang = true;
+      langC = true;
+      # cc-wrapper adds libstdc++ include paths only when this is set.
+      langCC = true;
+    };
 
     # icpx rejects these flags for the SPIR-V device target (spir64-unknown-unknown).
     hardeningUnsupportedFlags = [