Print log/error messages on stderr, not stdout

There are a couple of places where libc++ prints log/error messages to
stdout on its own. This may of course interfere with the output
generated with applications. Log/error messages should be directed to
stderr instead.

Differential Revision:	http://reviews.llvm.org/D8135
Reviewed by:	marshall

llvm-svn: 231767

libcxx/src/string.cpp

@@ -39,7 +39,7 @@ void throw_helper( const string& msg )
 #ifndef _LIBCPP_NO_EXCEPTIONS
     throw T( msg );
 #else
-    printf("%s\n", msg.c_str());
+    fprintf(stderr, "%s\n", msg.c_str());
     abort();
 #endif
 }