Enable and fix warnings during the build.

Although CMake adds warning flags, they are ignored in the libc++ headers
because the headers '#pragma system header' themselves.

This patch disables the system header pragma when building libc++ and fixes
the warnings that arose.

The warnings fixed were:
1. <memory> - anonymous structs are a GNU extension
2. <functional> - anonymous structs are a GNU extension.
3. <__hash_table> - Embedded preprocessor directives have undefined behavior.
4. <string> - Definition is missing noexcept from declaration.
5. <__std_stream> - Unused variable.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@242623 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -1936,7 +1936,12 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string()
 template <class _CharT, class _Traits, class _Allocator>
 inline _LIBCPP_INLINE_VISIBILITY
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const allocator_type& __a)
-    : __r_(__a)
+#if _LIBCPP_STD_VER <= 14
+        _NOEXCEPT_(is_nothrow_copy_constructible<allocator_type>::value)
+#else
+        _NOEXCEPT
+#endif
+: __r_(__a)
 {
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);