toolchains_llvm: x_compile.patch does not apply extra_cxx_flags to C++20 module actions

[phlax]

Context

toolchains_llvm/1.9.1.envoy was added in 81606e3 and is being rolled out via envoyproxy/envoy#47658. Both Envoy patches (allow_nonroot.patch, x_compile.patch) were reviewed against the upstream v1.9.0…v1.9.1 diff and are still required and correctly rebased. This gap fell out of that review.

x_compile.patch does not cover C++20 module actions

x_compile.patch adds cxx_cross_lib and injects per-target -nostdinc++ -isystem <libc++ include> via extra_cxx_flags. Upstream only wires extra_cxx_flags to @rules_cc//cc/toolchains/actions:cpp_compile_actions. The new cpp_modules feature in v1.9.1 adds CPP_MODULE_ACTIONS to the msan/nomsan cc_args, but our cross-lib flags are not applied to:

• cpp20_module_compile
• cpp20_module_codegen
• cpp_module_deps_scanning

Consequence: cross-compiling with --experimental_cpp_modules --features=cpp_modules would fail to find libc++ headers for PCM/scan actions. Not a problem for Envoy today (we don't enable C++20 named modules), but it needs to be recorded so it doesn't bite later.

Action:

• Add a note to MODULE.bazel and/or a preamble in x_compile.patch documenting the gap (text before the first diff line is ignored by patch; source.json patch sha must be regenerated).
• When/if we enable C++20 modules: extend the patch so the extra_cxx_flags cc_args also target CPP_MODULE_ACTIONS, and re-check the stdlib = "libc" auto-fallback for cross targets against the module actions.

References

• Registry module: https://github.com/envoyproxy/bazel-registry/tree/81606e3b499db57b46c5986224511c11729ad2ad/modules/toolchains_llvm/1.9.1.envoy
• Upstream diff: bazel-contrib/toolchains_llvm@v1.9.0...v1.9.1
• Upstream cc_toolchain_config.bzl CPP_MODULE_ACTIONS: https://github.com/bazel-contrib/toolchains_llvm/blob/v1.9.1/toolchain/cc_toolchain_config.bzl
• Envoy rollout: [WIP] deps: Bump @toolchains_llvm -> 1.9.1.envoy envoy#47658