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

[Louis Dionne via cfe-commits]

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

`libunwind` doesn't have the libc++ headers available.

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