libstdc++: Add missing noexcept to std::scoped_allocator_adaptor

libstdc++: Add missing noexcept to std::scoped_allocator_adaptor

The standard requires these constructors and accessors to be noexcept.

libstdc++-v3/ChangeLog:

	* include/std/scoped_allocator (scoped_allocator_adaptor): Add
	noexcept to all constructors except the default constructor.
	(scoped_allocator_adaptor::inner_allocator): Add noexcept.
	(scoped_allocator_adaptor::outer_allocator): Likewise.
	* testsuite/20_util/scoped_allocator/noexcept.cc: New test.

(cherry picked from commit b960c253e988c68ed3f3829125bc267bdf169356)