[libc++] Avoid name conflict with declaration in <unistd.h>

It appears that ::link is also declared as another kind of symbol
in <unistd.h>, which we seem to get transitively when we start using
a single top-level module. Instead of declaring these names as global
extern variables, use function parameters to achieve the same thing.

Author: ldionne

libcxx/test/libcxx/diagnostics/chrono.nodiscard.verify.cpp

@@ -21,11 +21,7 @@
 #include "test_macros.h"
 
 // These types have "private" constructors.
-extern std::chrono::time_zone tz;
-extern std::chrono::time_zone_link link;
-extern std::chrono::leap_second leap;
-
-void test() {
+void test(std::chrono::time_zone tz, std::chrono::time_zone_link link, std::chrono::leap_second leap) {
   std::chrono::tzdb_list& list = std::chrono::get_tzdb_list();
   list.front();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   list.begin();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}