[Ada] Spurious error on overloaded equality in postcondition

[Ada] Spurious error on overloaded equality in postcondition

This patch fixes a spurious error in a postcondition in a nested
instantiation when the expression includes an inherited equality and
checks are enabled.

2019-07-10  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Equality_Op): Do not replace the resolved
	operator by its alias if expander is not active, because the
	operand type may not be frozen yet and its inherited operations
	have not yet been created.

gcc/testsuite/

	* gnat.dg/equal8.adb, gnat.dg/equal8.ads,
	gnat.dg/equal8_pkg.ads: New testcase.

From-SVN: r273327