[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
diff --git a/libcxx/test/std/containers/associative/map/map.access/empty.fail.cpp b/libcxx/test/std/containers/associative/map/map.access/empty.fail.cpp
index 71b18c9..0043097 100644
--- a/libcxx/test/std/containers/associative/map/map.access/empty.fail.cpp
+++ b/libcxx/test/std/containers/associative/map/map.access/empty.fail.cpp
@@ -24,7 +24,7 @@
 int main(int, char**)
 {
     std::map<int, int> 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;
 }
diff --git a/libcxx/test/std/containers/associative/multimap/empty.fail.cpp b/libcxx/test/std/containers/associative/multimap/empty.fail.cpp
index 4aeba89..2492be5 100644
--- a/libcxx/test/std/containers/associative/multimap/empty.fail.cpp
+++ b/libcxx/test/std/containers/associative/multimap/empty.fail.cpp
@@ -24,7 +24,7 @@
 int main(int, char**)
 {
     std::multimap<int, int> 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;
 }
diff --git a/libcxx/test/std/containers/associative/multiset/empty.fail.cpp b/libcxx/test/std/containers/associative/multiset/empty.fail.cpp
index 742ea3f..abc67ed 100644
--- a/libcxx/test/std/containers/associative/multiset/empty.fail.cpp
+++ b/libcxx/test/std/containers/associative/multiset/empty.fail.cpp
@@ -24,7 +24,7 @@
 int main(int, char**)
 {
     std::multiset<int> 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;
 }
diff --git a/libcxx/test/std/containers/associative/set/empty.fail.cpp b/libcxx/test/std/containers/associative/set/empty.fail.cpp
index e181fd4..ba6aadc 100644
--- a/libcxx/test/std/containers/associative/set/empty.fail.cpp
+++ b/libcxx/test/std/containers/associative/set/empty.fail.cpp
@@ -24,7 +24,7 @@
 int main(int, char**)
 {
     std::set<int> 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;
 }