ada: Support new GNAT-specific aspect Ghost_Predicate

ada: Support new GNAT-specific aspect Ghost_Predicate

New aspect Ghost_Predicate allows the use of ghost entities in the
predicate expression, even if the type is not ghost itself. As a result,
subtypes with a ghost predicate cannot be used in membership tests.

Subtypes with ghost predicates are subject to the same additional
restrictions as subtypes with aspect Dynamic_Predicate.
They are governed for compilation by assertion policy Ghost.
Checking of the predicate itself is governed by the usual assertion
policy (Static_Predicate/Dynamic_Predicate/Predicate) independently
of the ghost predicate.

gcc/ada/

	* doc/gnat_rm/implementation_defined_aspects.rst: Document new
	aspect.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Whitespace.
	* aspects.adb (Init_Canonical_Aspect): Set it to Predicate.
	* aspects.ads: Set global constants for new aspect.
	* einfo.ads: Describe new flag related to new aspect.
	* exp_ch6.adb (Can_Fold_Predicate_Call): Do not fold new aspect.
	* exp_util.adb (Make_Predicate_Check): Add comment.
	* gen_il-fields.ads: Add new flag.
	* gen_il-gen-gen_entities.adb: Add new flag.
	* ghost.adb (Is_OK_Ghost_Context): Ghost predicate is an OK
	ghost context.
	(Mark_Ghost_Pragma): Add overloading with ghost mode parameter.
	* ghost.ads (Mark_Ghost_Pragma): Add overloading with ghpst mode
	parameter.
	(Name_To_Ghost_Mode): Make function public.
	* sem_aggr.adb: Issue error for violation of valid use.
	* sem_case.adb: Issue error for violation of valid use.
	* sem_ch13.adb: Adapt for new aspect.
	* sem_ch3.adb (Analyze_Full_Type_Declaration): Remove dead code
	which was trying to propagate Has_Predicates flag in the wrong
	direction (from derived to parent type).
	(Analyze_Number_Declaration): Issue error for violation of valid
	use.
	(Build_Derived_Type): Cleanup inheritance of predicate flags from
	parent to derived type.
	(Build_Predicate_Function): Only add a predicate check when it
	is not ignored as Ghost code.
	* sem_ch4.adb (Analyze_Membership_Op): Issue an error for use of
	a subtype with a ghost predicate as name in a membership test.
	* sem_ch5.adb (Check_Predicate_Use): Issue error for violation of
	valid use.
	* sem_eval.adb: Adapt code for Dynamic_Predicate to account for
	Ghost_Predicate too.
	* sem_prag.adb (Analyze_Pragma): Make pragma ghost or not.
	* sem_util.adb (Bad_Predicated_Subtype_Use): Adapt to new aspect.
	(Inherit_Predicate_Flags): Add inheritance of flag. Add parameter
	to apply to derived types.
	* sem_util.ads (Inherit_Predicate_Flags): Change signature.
	* snames.ads-tmpl: Add new aspect name.
	* gnat_rm.texi: Regenerate.