exp_ch6.adb (Expand_Call): Add comment on handling of back end intrinsic

exp_ch6.adb (Expand_Call): Add comment on handling of back end intrinsic

	* exp_ch6.adb (Expand_Call): Add comment on handling of back end
	intrinsic

	* exp_intr.adb (Expand_Intrinsic_Call): Ignore unrecognized intrinsic,
	leaving call unchanged.
	This deals with the case where the pragma Import specified
	an external name, to be handled by the back end.

	* sem_prag.adb (Process_Import_Or_Interface): Do not check validity of
	subprogram which is Imported with convention Intrinsic if an
	External_Name argument is specified.
	(Process_Import_Or_Interface): Properly diagnose link name argument.
	(Inlining_Not_Possible): New name for Cannot_Inline, to avoid confusion
	with Sem_Ch6.Cannot_Inline.
	(Process_Inline): Provide separate warning for inapplicable inline
	pragma.
	(Cannot_Inline): Reject subprograms that have an at_end handler, so that
	treatment is uniform on different targets.

From-SVN: r91882