[llvm] [LLVM] Add `LLVM_<proj>_RUNTIME_TARGETS` to set targets per-project (PR #81557)

[Petr Hosek via llvm-commits]

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@@ -236,6 +236,7 @@ function(runtime_default_target)
   endif()
 
   set_enable_per_target_runtime_dir()
+  set(LLVM_ENABLE_RUNTIMES ${ARG_RUNTIMES})
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petrhosek wrote:

Previously, we would passthrough `LLVM_ENABLE_RUNTIMES` as a global variable, but now we always pass in `RUNTIMES` as a function argument, and then set `LLVM_ENABLE_RUNTIMES` as a local variable shadowing the global one and rely on the passthrough mechanism.

That's quite obscure and potentially error prone, I think we should stop using the passthrough machinery and instead pass `LLVM_ENABLE_RUNTIMES` as an extra argument.

https://github.com/llvm/llvm-project/pull/81557