[Ada] Lock_Free aspect takes precedence over Ceiling_Locking locking policy

[Ada] Lock_Free aspect takes precedence over Ceiling_Locking locking policy

Allow the Lock_Free aspect of a protected type to be True (but only by
explicit specification) if Ceiling_Locking locking policy has been specified.
Clarify that the Lock_Free aspect takes precedence over the Ceiling_Locking
policy in that case.

gcc/ada/

	* sem_ch9.adb
	(Satisfies_Lock_Free_Requirements): If Ceiling_Locking locking
	policy has been specified, then either return False (if Lock_Free
	was not explicitly specified) or generate a warning that ceiling
	locking will not be implemented for this protected unit (if
	Lock_Free was explicitly specified). Generate an error message (in
	addition to returning False) if an explicit Lock_Free aspect
	specification is rejected because atomic primitives are not
	supported on the given target.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Clarify that the
	Lock_Free aspect for a protected unit takes precedence over the
	Ceiling_Locking locking policy in the case where both apply.
	* gnat_rm.texi: Regenerate.