[libc++] Remove official Clang 18 support. (#130142)

Since Clang 20 has been release we no longer support Clang 18 per our
policy.

Note the Clang 18 workarounds will be removed in a follow-up patch.

Author: mordante

.github/workflows/libcxx-build-and-test.yaml

@@ -100,10 +100,6 @@ jobs:
           - config: 'generic-cxx26'
             cc: 'clang-19'
             cxx: 'clang++-19'
-          # Release transition
-          - config: 'generic-cxx23'
-            cc: 'clang-18'
-            cxx: 'clang++-18'
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: ${{ matrix.config }}

libcxx/docs/index.rst

@@ -131,7 +131,7 @@ velocity, libc++ drops support for older compilers as newer ones are released.
 ============ =================== ========================== =====================
 Compiler     Versions            Restrictions               Support policy
 ============ =================== ========================== =====================
-Clang        18, 19, 20, 21-git                             latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
+Clang        19, 20, 21-git                                 latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
 AppleClang   15                                             latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
 Open XL      17.1.3 (AIX)                                   latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
 GCC          14                  In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_

libcxx/include/__configuration/compiler.h

@@ -33,8 +33,8 @@
 // Warn if a compiler version is used that is not supported anymore
 // LLVM RELEASE Update the minimum compiler versions
 #  if defined(_LIBCPP_CLANG_VER)
-#    if _LIBCPP_CLANG_VER < 1800
-#      warning "Libc++ only supports Clang 18 and later"
+#    if _LIBCPP_CLANG_VER < 1900
+#      warning "Libc++ only supports Clang 19 and later"
 #    endif
 #  elif defined(_LIBCPP_APPLE_CLANG_VER)
 #    if _LIBCPP_APPLE_CLANG_VER < 1500