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

[Joseph Huber 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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jhuber6 wrote:

I think I tired adding it to `CMAKE_ARGS` but it was being overridden by the passthrough version like you said. Changing `PASSTHROUGH` was a much bigger chance so I tried to keep it small. Maybe I could set `LLVM_ENABLE_RUNTIMES` to `""` to make it more obvious.

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