c++: retval dtor on rethrow [PR112301]

In r12-6333 for PR33799, I fixed the example in [except.ctor]/2.  In that
testcase, the exception is caught and the function returns again,
successfully.

In this testcase, however, the exception is rethrown, and hits two separate
cleanups: one in the try block and the other in the function body.  So we
destroy twice an object that was only constructed once.

Fortunately, the fix for the normal case is easy: we just need to clear the
"return value constructed by return" flag when we do it the first time.

This gets more complicated with the named return value optimization, since
we don't want to destroy the return value while the NRV variable is still in
scope.

	PR c++/112301
	PR c++/102191
	PR c++/33799

gcc/cp/ChangeLog:

	* except.cc (maybe_splice_retval_cleanup): Clear
	current_retval_sentinel when destroying retval.
	* semantics.cc (nrv_data): Add in_nrv_cleanup.
	(finalize_nrv): Set it.
	(finalize_nrv_r): Fix handling of throwing cleanups.

gcc/testsuite/ChangeLog:

	* g++.dg/eh/return1.C: Add more cases.

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