libstdc++: Fix __max_diff_type::operator>>= for negative values

libstdc++: Fix __max_diff_type::operator>>= for negative values

This patch fixes sign bit propagation when right-shifting a negative
__max_diff_type value by more than one, a bug that our existing test
coverage didn't expose until r14-159-g03cebd304955a6 fixed the front
end's 'signed typedef-name' handling that the test relies on (which is
a non-standard extension to the language grammar).

libstdc++-v3/ChangeLog:

	* include/bits/max_size_type.h (__max_diff_type::operator>>=):
	Fix propagation of sign bit.
	* testsuite/std/ranges/iota/max_size_type.cc: Avoid using the
	non-standard 'signed typedef-name'.  Add some compile-time tests
	for right-shifting a negative __max_diff_type value by more than
	one.