Reject components in extensions overlapping with the parent

Reject components in extensions overlapping with the parent

Such problematic components can be specified by means of a component
clause but they cannot be fully supported by the type system.  They
had initially been forbidden, then we decided to accept them by working
around the type system, but this is very fragile and, for example, any
static aggregate is guaranteed to trigger an ICE with the current
implementation.

We now reject them again, except if the -gnatd.K switch is passed.

gcc/ada/ChangeLog:
	* debug.adb (d.K): Document new usage.
	* fe.h (Debug_Flag_Dot_KK): Declare.
	* gcc-interface/decl.c (gnat_to_gnu_field): Give an error when the
	component overlaps with the parent subtype, except with -gnatd.K.