[llvm] [libcxx] Add libc++ github actions workflow to replace buildkite (PR #71836)

[Louis Dionne via llvm-commits]

================
@@ -0,0 +1,112 @@
+name: Build and Test libc++
+
+on: pull_request
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+
+env:
+  CMAKE: "/opt/bin/cmake"
+
+# Comment
+jobs:
+  stage1:
+    runs-on: libcxx-runners-16
+    continue-on-error: false
+    strategy:
+      fail-fast: true
+      matrix:
+        config: [ 'generic-cxx26', 'generic-cxx03', 'generic-modules' ]
+        cc: [ 'clang-18' ]
----------------
ldionne wrote:

Instead of hardcoding `clang-18` here, we should define env variables like

```
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_HEAD_VERSION: "18"   # Used compiler, update POST-BRANCH.
    GCC_STABLE_VERSION: "13"
```

and then we should use `clang-${LLVM_HEAD_VERSION}` or whatever makes the most sense with GH actions.


https://github.com/llvm/llvm-project/pull/71836