Remove redundant static assertions in [meta.unary.prop] traits

Remove redundant static assertions in [meta.unary.prop] traits

The type property predicates that are implemented by a compiler builtin
already do the right checks in the compiler. The checks for complete
type or unbounded arrays were wrong for these types anyway.

	* include/std/type_traits (is_empty, is_polymorphic, is_final)
	(is_abstract, is_aggregate): Remove static_assert.
	* testsuite/20_util/is_abstract/incomplete_neg.cc: Check for error
	from builtin only.
	* testsuite/20_util/is_aggregate/incomplete_neg.cc: Likewise. Add
	missing -std=gnu++17 option.
	* testsuite/20_util/is_empty/incomplete_neg.cc: New test.
	* testsuite/20_util/is_final/incomplete_neg.cc: New test.
	* testsuite/20_util/is_polymorphic/incomplete_neg.cc: Check for error
	from builtin only.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@272000 138bc75d-0d04-0410-961f-82ee72b054a4