[Ada] Crash on subprogram instantiation in nested package

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch fixes a crash on a subpogram instance that appears within a package
that declares the actual type for the instance, when the corresponding type is
a private or incomplete formal type.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada

2017-12-15  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch6.adb (Possible_Freeze): Do not set Delayed_Freeze on an
	subprogram instantiation, now that the enclosing wrapper package
	carries an explicit freeze node. THis prevents freeze nodes for the
	subprogram for appearing in the wrong scope. This is relevant when the
	generic subprogram has a private or incomplete formal type and the
	instance appears within a package that declares the actual type for the
	instantiation, and that type has itself a delayed freeze.

gcc/testsuite/

2017-12-15  Ed Schonberg  <schonberg@adacore.com>

	* gnat.dg/subp_inst.adb, gnat.dg/subp_inst_pkg.adb,
	gnat.dg/subp_inst_pkg.ads: New testcase.

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