Give A an explicitly non-throwing destructor so that B's destructor is

itself non-throwing.  Since nested_exception's destructor is
non-throwing, if B's destructor is not, this causes an error in C++03
mode due to the overriding function having a more lax specification.
This did not occur in C++0x mode as A's destructor was implicitly
non-throwing.

llvm-svn: 135400