Implement P0599: 'noexcept for hash functions'. Fix a couple of hash functions (optional<T> and unique_ptr<T>) which were mistakenly marked as 'noexcept'. Reviewed as https://reviews.llvm.org/D31234

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@298573 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/string.view/string.view.hash/string_view.pass.cpp

@@ -23,6 +23,8 @@
 #include <cassert>
 #include <type_traits>
 
+#include "test_macros.h"
+
 using std::string_view;
 
 template <class SV>
@@ -36,6 +38,7 @@ test()
     typedef typename SV::value_type char_type;
     typedef std::basic_string<char_type> String;
     typedef std::hash<String> SH;
+    ASSERT_NOEXCEPT(H()(SV()));
 
     char_type g1 [ 10 ];
     char_type g2 [ 10 ];