Fixes #3317.

Fix is_virtual_base_of so it doesn't lose exception specifications on it's destructors (causes GCC and maybe other compilers to choke).

[SVN r55428]

libs/type_traits

@@ -1 +1 @@
-Subproject commit c9e77b9c8d321b6e104d46b6b40c0f8b04842cf1
+Subproject commit f1817aeee12177bc6cac1532cecfb8806c5a5975