clean up some qualifier casting

This cleans up the `-Wqual-cast` warnings from gcc 6 when building
libc++.  NFC.

llvm-svn: 290789

libcxx/include/__std_stream

@@ -297,7 +297,7 @@ __stdoutbuf<_CharT>::overflow(int_type __c)
                         return traits_type::eof();
                     if (__r == codecvt_base::partial)
                     {
-                        pbase = (char_type*)__e;
+                        pbase = const_cast<char_type*>(__e);
                     }
                 }
                 else

libcxx/include/type_traits

@@ -443,7 +443,8 @@ inline _LIBCPP_NO_CFI _LIBCPP_INLINE_VISIBILITY
 _Tp*
 addressof(_Tp& __x) _NOEXCEPT
 {
-    return (_Tp*)&reinterpret_cast<const volatile char&>(__x);
+  return reinterpret_cast<_Tp *>(
+      const_cast<char *>(&reinterpret_cast<const volatile char &>(__x)));
 }
 
 #endif // _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF