[libc++][CI] Upgrades to LLVM 18 as HEAD version.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D156922

Author: mordante

libcxx/utils/ci/buildkite-pipeline.yml

@@ -26,8 +26,8 @@ env:
     # LLVM POST-BRANCH bump version
     # LLVM POST-BRANCH add compiler test for ToT - 1, e.g. "Clang 17"
     # LLVM RELEASE bump remove compiler ToT - 3, e.g. "Clang 15"
-    LLVM_STABLE_VERSION: "16"
-    LLVM_HEAD_VERSION: "17"
+    LLVM_STABLE_VERSION: "16" # Used for tooling, update after the RELEASE.
+    LLVM_HEAD_VERSION: "18"   # Used compiler, update POST-BRANCH.
     GCC_STABLE_VERSION: "12"
 steps:
   #
@@ -335,6 +335,24 @@ steps:
           limit: 2
     timeout_in_minutes: 120
 
+  - label: "Clang 17"
+    command: "libcxx/utils/ci/run-buildbot generic-cxx23"
+    artifact_paths:
+      - "**/test-results.xml"
+      - "**/*.abilist"
+    env:
+        CC: "clang-17"
+        CXX: "clang++-17"
+        ENABLE_CLANG_TIDY: "On"
+    agents:
+      queue: "libcxx-builders"
+      os: "linux"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    timeout_in_minutes: 120
+
   # Tests with the sanitizers.
   - group: "Sanitizers"
     steps: