The current std::list::merge code calls size() before starting to merge
any elements, so that the _M_size members can be updated after the merge
finishes. The work is done in a try-block so that the sizes can still be
updated in an exception handler if any element comparison throws.

The _M_size members only exist for the cxx11 ABI, so the initial call to
size() and the try-catch are only needed for that ABI. For the old ABI
the size() call performs an O(N) list traversal to get a value that
isn't even used, and catching exceptions just to rethrow them isn't
needed either.

This refactors the merge functions to remove the try-catch block and use
an RAII type instead. For the cxx11 ABI that type's destructor updates
the list sizes, and for the old ABI it's a no-op.

libstdc++-v3/ChangeLog:

	* include/bits/list.tcc (list::merge): Remove call to size() and
	try-catch block. Use _Finalize_merge instead.
	* include/bits/stl_list.h (list::_Finalize_merge): New
	scope guard type to update _M_size members after a merge.

Tested x86_64-linux. Committed to trunk.