[libc++] Disables -Wweak-vtables diagnostics.

This is a preparation to use Clang HEAD in the CI.

Author: mordante

libcxx/include/__expected/bad_expected_access.h

@@ -27,6 +27,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Err>
 class bad_expected_access;
 
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables")
 template <>
 class bad_expected_access<void> : public exception {
 protected:
@@ -58,6 +60,7 @@ class bad_expected_access : public bad_expected_access<void> {
 private:
   _Err __unex_;
 };
+_LIBCPP_DIAGNOSTIC_POP
 
 _LIBCPP_END_NAMESPACE_STD