[Ada] Skip postponed validation checks with compilation errors

[Ada] Skip postponed validation checks with compilation errors

If there are compilation errors then gigi doesn't backannotate the AST
with sizes, alignment, etc. The postponed compilation validation checks
can then easily crash or give spurious errors. We now just skip them.

gcc/ada/

	* gnat1drv.adb (Gnat1drv): Skip postponed checks when there are
	errors.