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

[Mehdi Amini via llvm-commits]

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@@ -231,6 +245,10 @@ linux_projects_to_test=$(exclude-linux $(compute-projects-to-test ${modified_pro
 linux_check_targets=$(check-targets ${linux_projects_to_test} | sort | uniq)
 linux_projects=$(add-dependencies ${linux_projects_to_test} | sort | uniq)
 
+linux_runtimes_to_test=$(compute-runtimes-to-test ${linux_projects_to_test})
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joker-eph wrote:

I was trying to understand why my PR which touches LLVM and MLIR but not clang triggered all the runtime tests, and ended up here.

It's not clear to me why is the `compute-runtimes-to-test` computed from the `linux_projects_to_test` and not from the `modified_projects`?
That is when clang is added to `linux_projects_to_test` because something else is modified (like LLVM), why do we need to test the runtimes?

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