[clang] [llvm] [clang][ci] Move libc++ testing into the main PR pipeline (PR #93318)

[Louis Dionne via cfe-commits]

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@@ -54,3 +54,68 @@ cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
 echo "--- ninja"
 # Targets are not escaped as they are passed as separate arguments.
 ninja -C "${BUILD_DIR}" -k 0 ${targets}
+
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ldionne wrote:

I don't think it makes sense to serialize all these different jobs and put them in the same script. Doing this has several downsides:
1. It's not parallelized anymore
2. If one job fails, you don't get signal from whether the other jobs would fail too
3. The reporting on Github is going to be confusing because everything will be under the same job
4. This doesn't scale if we want to add more testing to be done against the just-built Clang

Instead, I would suggest changing `generate-buildkite-pipeline` so that it adds additional steps to the pipeline when we're testing Clang. These steps can communicate via artifacts uploading/downloading like they used to.

https://github.com/llvm/llvm-project/pull/93318