[Ada] Missing error on tagged type conversion

[Ada] Missing error on tagged type conversion

The compiler does not report an error on a type conversion to/from a
tagged type whose parent type is an interface type and there is no
relationship between the source and target types. This bug has been
dormant since January/2016.

This patch also improves the text of errors reported on interface type
conversions suggesting how to fix these errors.

gcc/ada/

	* sem_res.adb (Resolve_Type_Conversion): Code cleanup since the
	previous static check has been moved to Valid_Tagged_Conversion.
	(Valid_Tagged_Conversion): Fix the code checking conversion
	to/from interface types since incorrectly returns True when the
	parent type of the operand type (or the target type) is an
	interface type; add missing static checks on interface type
	conversions.