libstdc++: Do not use SFINAE for propagate_const conversions [PR107525]

libstdc++: Do not use SFINAE for propagate_const conversions [PR107525]

As the PR notes, the current conversion operators are defined as
function templates so that we can use SFINAE. But this changes how they
are considered for overload resolution. This moves those operators into
base classes that can be specialized so the operators are obsent unless
the constraints are satisfied.

libstdc++-v3/ChangeLog:

	PR libstdc++/107525
	* include/experimental/propagate_const (operator element_type*()):
	Move into base class that can be partially specilized to iompose
	constraints.
	(operator const element_type*()): Likewise.
	* testsuite/experimental/propagate_const/observers/107525.cc: New test.