[libc++] Use Clang-16 for c++17.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D132788

Author: mordante

libcxx/utils/ci/buildkite-pipeline.yml

@@ -192,11 +192,8 @@ steps:
       - "**/test-results.xml"
       - "**/*.abilist"
     env:
-        # TODO (mordante) use head
-        #CC: "clang-${LLVM_HEAD_VERSION}"
-        #CXX: "clang++-${LLVM_HEAD_VERSION}"
-        CC: "clang-15"
-        CXX: "clang++-15"
+        CC: "clang-${LLVM_HEAD_VERSION}"
+        CXX: "clang++-${LLVM_HEAD_VERSION}"
     agents:
       queue: "libcxx-builders"
       os: "linux"