[libc++] Only include <setjmp.h> from the C library if it exists (#81887)

In 2cea1ba, we removed the <setjmp.h> header provided by libc++. However, we did not conditionally include the underlying <setjmp.h>
header only if the C library provides one, which we otherwise do consistently (see e.g. 647ddc0).

rdar://122978778

Author: ldionne

libcxx/include/csetjmp

@@ -33,7 +33,13 @@ void longjmp(jmp_buf env, int val);
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 
-#include <setjmp.h>
+// <setjmp.h> is not provided by libc++
+#if __has_include(<setjmp.h>)
+#  include <setjmp.h>
+#  ifdef _LIBCPP_SETJMP_H
+#    error "If libc++ starts defining <setjmp.h>, the __has_include check should move to libc++'s <setjmp.h>"
+#  endif
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header