Fix corner case issue with discriminated record type

Fix corner case issue with discriminated record type

The compiler generates code that writes too much data into a component
of a record subject to a representation clause, when the source of the
assignment is a call to a function that returns a discriminated record
type with default discriminants, variable size and a statically known
upper bound for this size, and the size of the component given by the
representation clause is lower than the value of this bound rounded up
to the alignment.

gcc/ada/ChangeLog:
	* gcc-interface/trans.c (Call_to_gnu): Also create a temporary for
	the return value if the LHS is a bit-field and the return type is
	a type padding a self-referential type.
	(gnat_to_gnu): Do not remove the padding on the result if it is too
	small with regard to the natural padding size.