libstdc++: Add a test checking for chrono::duration overflows

This test fails if chrono::days::rep or chrono::years::rep is a 32-bit
type, because a large days or years value silently overflows a 32-bit
integer when converted to seconds. It would be conforming to implement
chrono::days as chrono::duration<int32_t, ratio<86400>>, but would make
this overflow case more likely. Similarly for chrono::years,
chrono::months and chrono::weeks. This test is here to remind us not to
make that change lightly.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/duration/arithmetic/overflow_c++20.cc: New
	test.

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