[llvm] [openmp] [OpenMP] Fix runtimes default build (PR #149871)

[via llvm-commits]

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@@ -11,7 +11,7 @@
 include(ExtendPath)
 
 # The generated headers will be placed in clang's resource directory if present.
-if(OPENMP_STANDALONE_BUILD OR NOT LLVM_RUNTIMES_BUILD)
+if(NOT LLVM_TREE_AVAILABLE)
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jprotze wrote:

When building openmp as llvm project, `LLVM_TREE_AVAILABLE` is undefined, so the wrong branch will be used

https://github.com/llvm/llvm-project/pull/149871