d: Fix ICE in TypeInfoDeclaration, at dmd/declaration.c (PR100967)

d: Fix ICE in TypeInfoDeclaration, at dmd/declaration.c (PR100967)

Generate a stub TypeInfo class even if the root Object class is missing.
The front-end will take care of issuing an error and abort the
compilation when running semantic on constructed TypeInfo objects.

The errors issued by the code generation pass relating to missing or
disabled RTTI has been consolidated into a single function, so that a
meaningful error will be emitted before the front-end terminates.

gcc/d/ChangeLog:

	PR d/100967
	* d-frontend.cc (getTypeInfoType): Move TypeInfo checks to
	check_typeinfo_type and call new function.
	* d-tree.h (check_typeinfo_type): Declare.
	* typeinfo.cc: Include dmd/scope.h.
	(create_frontend_tinfo_types): Generate front-end types even if Object
	is missing.
	(build_typeinfo): Move TypeInfo checks to check_typeinfo_type and call
	new function.
	(check_typeinfo_type): New function.

gcc/testsuite/ChangeLog:

	PR d/100967
	* gdc.dg/pr100967.d: New test.