[libc++][WIP] Move to Github hosted builders

Libc++ experiences periodic outages of our Github actions builders,
which are quite disruptive. Unfortunately, the Github self-hosted
runner setup is very opaque. It works extremely well when it works,
but most contributors are unable to troubleshoot issues related to
the setup when they happen.

This patch attempts to move the libc++ builders to normal Github
hosted runners. This is primarily to evaluate the feasibility of
such a change, which would simplify our CI setup but may not scale
sufficiently for our needs.

Author: ldionne

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

@@ -49,7 +49,13 @@ env:
 jobs:
   stage1:
     if: github.repository_owner == 'llvm'
-    runs-on: libcxx-runners-8-set
+    runs: &docker-image
+      using: 'docker'
+      image: 'libcxx/utils/ci/Dockerfile'
+      args:
+        - BASE_IMAGE: ghcr.io/actions/actions-runner:2.317.0
+        - GCC_LATEST_VERSION: 14
+        - LLVM_HEAD_VERSION: 20
     continue-on-error: false
     strategy:
       fail-fast: false
@@ -84,7 +90,8 @@ jobs:
             **/crash_diagnostics/*
   stage2:
     if: github.repository_owner == 'llvm'
-    runs-on: libcxx-runners-8-set
+    runs:
+      <<: *docker-image
     needs: [ stage1 ]
     continue-on-error: false
     strategy:
@@ -136,12 +143,15 @@ jobs:
       matrix:
         config: [
           'generic-abi-unstable',
+          'generic-asan',
+          'generic-cxx26',
           'generic-hardening-mode-debug',
           'generic-hardening-mode-extensive',
           'generic-hardening-mode-fast',
           'generic-hardening-mode-fast-with-abi-breaks',
           'generic-merged',
           'generic-modules-lsv',
+          'generic-msan',
           'generic-no-exceptions',
           'generic-no-experimental',
           'generic-no-filesystem',
@@ -155,25 +165,15 @@ jobs:
           'generic-no-rtti',
           'generic-optimized-speed',
           'generic-static',
+          'generic-tsan',
+          'generic-ubsan',
           # TODO Find a better place for the benchmark and bootstrapping builds to live. They're either very expensive
           # or don't provide much value since the benchmark run results are too noise on the bots.
           'benchmarks',
           'bootstrapping-build'
         ]
-        machine: [ 'libcxx-runners-8-set' ]
-        include:
-        - config: 'generic-cxx26'
-          machine: libcxx-runners-8-set
-        - config: 'generic-asan'
-          machine: libcxx-runners-8-set
-        - config: 'generic-tsan'
-          machine: libcxx-runners-8-set
-        - config: 'generic-ubsan'
-          machine: libcxx-runners-8-set
-        # Use a larger machine for MSAN to avoid timeout and memory allocation issues.
-        - config: 'generic-msan'
-          machine: libcxx-runners-8-set
-    runs-on: ${{ matrix.machine }}
+    runs:
+      <<: *docker-image
     steps:
       - uses: actions/checkout@v4
       - name: ${{ matrix.config }}