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/memory

@@ -3420,7 +3420,7 @@ struct __scalar_hash<_Tp, 2>
             {
                 size_t __a;
                 size_t __b;
-            };
+            } __s;
         } __u;
         __u.__t = __v;
         return __murmur2_or_cityhash<size_t>()(&__u, sizeof(__u));
@@ -3442,7 +3442,7 @@ struct __scalar_hash<_Tp, 3>
                 size_t __a;
                 size_t __b;
                 size_t __c;
-            };
+            } __s;
         } __u;
         __u.__t = __v;
         return __murmur2_or_cityhash<size_t>()(&__u, sizeof(__u));
@@ -3465,7 +3465,7 @@ struct __scalar_hash<_Tp, 4>
                 size_t __b;
                 size_t __c;
                 size_t __d;
-            };
+            } __s;
         } __u;
         __u.__t = __v;
         return __murmur2_or_cityhash<size_t>()(&__u, sizeof(__u));