[clang] [test][Driver][clang] Fix darwin-embedded-search-paths.c when CLANG_DEFAULT_CXX_STDLIB is libc++ (PR #122145)

[Ian Anderson via cfe-commits]

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@@ -1,3 +1,4 @@
+// REQUIRES: default-cxx-stdlib=libstdc++
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ian-twilightcoder wrote:

Sometimes `default-cxx-stdlib` isn't set at all, and so the `REQUIRES` doesn't always work. `REQUIRES` doesn't have a `!=`, but you can do `!(a=b)`. You can also do `!a || a=b`, but it's probably best to cover libc++ and "everything else" instead of just libc++/libstdc++.

https://github.com/llvm/llvm-project/pull/122145