[libc++] Require the use of clang-verify in .fail.cpp tests that don't fail without it

Some tests do not fail at all when -verify is not supported, unless some
arbitrary warning flag is added to make them fail. We currently used
-Werror=unused-result to make them fail, but doing so makes the test
suite a lot more inscrutable. It seems better to just disable those
tests when -verify is not supported.

Differential Revision: https://reviews.llvm.org/D76256

libcxx/test/std/strings/string.view/string.view.capacity/empty.fail.cpp

@@ -15,6 +15,7 @@
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
 // UNSUPPORTED: clang-3.3, clang-3.4, clang-3.5, clang-3.6, clang-3.7, clang-3.8
+// REQUIRES: verify-support
 
 #include <string_view>
 
@@ -25,5 +26,5 @@ int main(int, char**)
     std::string_view c;
     c.empty();  // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
 
-  return 0;
+    return 0;
 }