libstdc++: Remove try/catch overhead in std::variant::emplace

The __variant_construct_by_index helper function sets the new index
before constructing the new object. This means that if the construction
throws then the exception needs to be caught, so the index can be reset
to variant_npos, and then the exception rethrown. This means callers are
responsible for restoring the variant's invariants and they need the
overhead of a catch handler and a rethrow.

If we don't set the index until after construction completes then the
invariant is never broken, and callers can ignore the exception and let
it propagate. The callers all call _M_reset() first, which sets index to
variant_npos as required while the variant is valueless.

We need to be slightly careful here, because changing the order of
operations in __variant_construct_by_index and removing the try-block
from variant::emplace<I> changes an implicit ABI contract between those
two functions. If the linker were to create an executable containing an
instantiation of the old __variant_construct_by_index and an
instantiation of the new variant::emplace<I> code then we would have a
combination that breaks the invariant and doesn't have the exception
handling to restore it. To avoid this problem, we can rename the
__variant_construct_by_index function so that the new emplace<I> code
calls a new symbol, and is unaffected by the behaviour of the old
symbol.

libstdc++-v3/ChangeLog:

	* include/std/variant (__detail::__variant::__get_storage):
	Remove unused function.
	(__variant_construct_by_index): Set index after construction is
	complete. Rename to ...
	(__detail::__variant::__construct_by_index): ... this.
	(variant): Use new name for __variant_construct_by_index friend
	declaration. Remove __get_storage friend declaration.
	(variant::emplace): Use new name and remove try-blocks.

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