[clang] [clang] prevent double package search if LLVM already found in parent directory (PR #218965)

[via cfe-commits]

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@@ -45,7 +45,9 @@ if(CLANG_BUILT_STANDALONE)
     mark_as_advanced(LLVM_ENABLE_ASSERTIONS)
   endif()
 
-  find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_DIR}")
+  if(NOT LLVM_FOUND)
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zhangweize9-cyber wrote:

At the time, I was only thinking about the possibility that the `LLVM_DIR` variable might be contaminated by environmental factors. 

Additionally, if you write two conditions at the same time, the former is a scope variable and the latter is a cached variable, which will cause the condition to return true directly.

https://github.com/llvm/llvm-project/pull/218965