[libc++] Do not force the use of -Werror in verify tests

Forcing -Werror and other warnings means that the test suite isn't
actually testing what most people are seeing in their code -- it seems
better and less arbitrary to compile these tests as close as possible
to the compiler default instead.

Removing -Werror also means that we get to differentiate between
diagnostics that are errors and those that are warnings, which makes
the test suite more precise.

Differential Revision: https://reviews.llvm.org/D76311

libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp

@@ -20,7 +20,6 @@ int main(int, char**) {
     // expected-error@array:* {{to_array does not accept multidimensional arrays}}
     // expected-error@array:* {{to_array requires copy constructible elements}}
     // expected-error@array:* 3 {{cannot initialize}}
-    // expected-error@array:* 0+ {{suggest braces}}
     std::to_array(source); // expected-note {{requested here}}
   }