[Ada] Entity names are not unique

[Ada] Entity names are not unique

This patch updates the Unique_Name procedure in order to prefix the
string "ada___" to child units that have a nested subprogram or package,
so that they do not clash with a parent package of the same name.

This is for GNATprove only and does not affect regular compilation.

2019-07-10  Simon Buist  <buist@adacore.com>

gcc/ada/

	* sem_util.ads (Child_Prefix): New constant.
	* sem_util.adb (Unique_Name): Add a special prefix to child
	units that have a nested subprogram or package.

From-SVN: r273343