[libcxx] [libunwind] [libc++] Harden the test suite for running against older libc++ versions (PR #214820)

[Nikolas Klauser via cfe-commits]

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@@ -13,7 +13,7 @@
 #ifdef __has_include
 #  if __has_include(<version>)
 #    include <version>
-#  else
+#  elif __has_include(<ciso646>)
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philnik777 wrote:

Why did this change? I'm not aware of any implementation that didn't have `<ciso646>`.

https://github.com/llvm/llvm-project/pull/214820