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.

llvm-svn: 242623
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 87a82490fc6b89fd3dfe033054e1bdc6b87fd9f4
diff --git a/include/functional b/include/functional
index 5ee9417..9bed783 100644
--- a/include/functional
+++ b/include/functional
@@ -2402,14 +2402,14 @@
                 size_t __b;
                 size_t __c;
                 size_t __d;
-            };
+            } __s;
         } __u;
-        __u.__a = 0;
-        __u.__b = 0;
-        __u.__c = 0;
-        __u.__d = 0;
+        __u.__s.__a = 0;
+        __u.__s.__b = 0;
+        __u.__s.__c = 0;
+        __u.__s.__d = 0;
         __u.__t = __v;
-        return __u.__a ^ __u.__b ^ __u.__c ^ __u.__d;
+        return __u.__s.__a ^ __u.__s.__b ^ __u.__s.__c ^ __u.__s.__d;
 #elif defined(__x86_64__)
         // Zero out padding bits
         union
@@ -2419,12 +2419,12 @@
             {
                 size_t __a;
                 size_t __b;
-            };
+            } __s;
         } __u;
-        __u.__a = 0;
-        __u.__b = 0;
+        __u.__s.__a = 0;
+        __u.__s.__b = 0;
         __u.__t = __v;
-        return __u.__a ^ __u.__b;
+        return __u.__s.__a ^ __u.__s.__b;
 #else
         return __scalar_hash<long double>::operator()(__v);
 #endif