[Ada] Incorrect code for -gnateV switch

[Ada] Incorrect code for -gnateV switch

This patch corrects the code generated by the -gnateV switch in the case
of a private type whose full type is a modular type, removing spurious
run-time failures.

In addition, this corrects the initialization of exception occurrences
in exception handlers to avoid leaving data uninitialized, which caused
-gnateV to raise spurious errors.

2019-08-19  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_attr.adb (Attribute_Valid): Correct the handling of
	private types where the full type is modular. System.Address is
	an example. Otherwise, we convert uncheckedly to a signed type,
	so we get an incorrect range 0 .. -1, for which all values will
	fail.  The 'Valid attribute is illegal for such types, but we
	generate such illegal attribute_references for 'Valid_Scalars,
	and we generate 'Valid_Scalars when the -gnateV switch is used.
	Rename Btyp --> PBtyp to avoid hiding the outer Btyp, which was
	confusing.
	* libgnat/a-except.adb: Set the Exception_Raised component.
	Otherwise, we have incorrect reads of invalid data.

gcc/testsuite/

	* gnat.dg/valid_scalars2.adb: New testcase.

From-SVN: r274660