[libc++][hardening] Make the safe mode pipeline run with modules enabled

[Mordante]

Thanks, LGTM!

[var-const]

Rebase

[var-const]

The CI errors (https://buildkite.com/llvm-project/libcxx-ci/builds/29641#018a8bd6-1a55-4423-a6c2-042dcdd8e11d) are unrelated -- they are caused by https://reviews.llvm.org/D156225 and will be fixed by https://reviews.llvm.org/D159509. Will proceed with the merge.

libcxx/cmake/caches/Generic-safe-mode.cmake

+set(LIBCXX_ENABLE_STD_MODULES ON CACHE BOOL "") # TODO MODULES Remove when enabled automatically.
 set(LIBCXX_HARDENING_MODE "safe" CACHE STRING "")

libcxx/utils/ci/buildkite-pipeline.yml

- label: "Hardened mode"
     timeout_in_minutes: 120
 
   - label: "Safe mode"
     command: "libcxx/utils/ci/run-buildbot generic-safe-mode"
     artifact_paths:
       - "**/test-results.xml"
       - "**/*.abilist"
     env:
-        CC: "clang-${LLVM_HEAD_VERSION}"
-        CXX: "clang++-${LLVM_HEAD_VERSION}"
+        # Note: Modules require an absolute path for `clang-scan-deps` (see
+        # https://github.com/llvm/llvm-project/issues/61006).
+        CC: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang"
+        CXX: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang++"
+        CMAKE: "/opt/bin/cmake"
         ENABLE_CLANG_TIDY: "On"
     agents:
       queue: "libcxx-builders"
       os: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2