PR libstdc++/80229 restore support for shared_ptr<function type>

PR libstdc++/80229 restore support for shared_ptr<function type>

	PR libstdc++/80229
	* include/bits/shared_ptr_base.h
	(__shared_ptr::_M_enable_shared_from_this_with): Change parameters to
	non-const and then use remove_cv to get unqualified type.
	* testsuite/20_util/enable_shared_from_this/members/const.cc: Don't
	cast away constness on object created const.
	* testsuite/20_util/shared_ptr/cons/80229.cc: New test.

From-SVN: r246520