[C++0x] std::exception_ptr rvalues

[Jonathan Wakely]

n2884 removed support for swapping with rvalues everywhere in the
library, but I missed exception_ptr::swap() when I changed libstdc++.
The symbol is not exported so it can just be removed.

exception_ptr's move assignment operator doesn't actually move right
now:  a temporary is copy-constructed then swapped with *this.  This
patch casts the source to an rvalue-reference, ensuring the temporary
is move-constructed instead, avoiding a reference count update.

        * libsupc++/exception_ptr.h (exception_ptr::swap(exception_ptr&&)):
        Remove.
        (exception_ptr::operator=(exception_ptr&&)): Cast source to
        rvalue-reference so that move constructor is called.
        * testsuite/18_support/exception_ptr/move.cc: New.

As noted in the new file, the test checks for behaviour that is not
required, but is a property of this implementation that I think should
be tested.

Tested x86_64-unknown-linux-gnu, OK for trunk?  Should this go on the
4.4. branch too?

Jonathan
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