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

[Michał Górny via cfe-commits]

mgorny wrote:

This `find_package()` is hit only if `CLANG_BUILT_STANDALONE` is set. `CLANG_BUILT_STANDALONE` is only set if building with clang as the top-level directory. How can LLVM be found then?

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