[Ada] Fix assertion failures with -gnatde and -gnatdv

[Pierre-Marie de Rodat]

This fixes a couple of assertion failures when using -gnatde and -gnatdv
on a compiler built with assertions enabled.  No functional changes.

Tested on x86_64-pc-linux-gnu, committed on trunk

2018-12-03  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* einfo.adb (Write_Entity_Info): Don't take Scope of Standard
	package.
	* sem_ch4.adb (Remove_Abstract_Operations): Add missing blank
	lines to -gnatdv output.
	* sem_type.adb (Write_Overloads): Take Entity of entity names
	only.
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