[Ada] Crash on ghost assignment check for illegal code

[Ada] Crash on ghost assignment check for illegal code

gcc/ada/

	* errout.adb (Error_Msg_NEL): Do not call Set_Posted if errors
	are being ignored.
	(Error_Msg): Change Errors_Must_Be_Ignored to use the getter.
	* sem_ch8.adb (Find_Direct_Name): Do not skip all the error
	checks when ignoring errors, but instead do not add an entry to
	the Urefs table if errors are being ignored.
	* exp_ch5.adb: Minor comment fix.