[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/strings/basic.string/string.capacity/empty.fail.cpp

@@ -24,7 +24,7 @@
 int main(int, char**)
 {
     std::string c;
-    c.empty();  // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+    c.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 
     return 0;
 }