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/__std_stream

@@ -276,7 +276,6 @@ __stdoutbuf<_CharT>::overflow(int_type __c)
             codecvt_base::result __r;
             char_type* pbase = &__1buf;
             char_type* pptr = pbase + 1;
-            char_type* epptr = pptr;
             do
             {
                 const char_type* __e;