[Ada] Fix a freezing issue

[Ada] Fix a freezing issue

2018-07-31  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem.ads (Inside_Preanalysis_Without_Freezing): New global
	counter.
	* sem.adb (Semantics): This subprogram has now the
	responsibility of resetting the counter before analyzing a unit,
	and restoring its previous value before returning.
	* freeze.adb (Freeze_Entity): Do not freeze if we are
	preanalyzing without freezing.
	* sem_res.adb (Preanalyze_And_Resolve): Set & restore
	In_Preanalysis_Without_Freezing.

From-SVN: r263091