[Ada] Keep assertions in internal units enabled for GNATprove

[Ada] Keep assertions in internal units enabled for GNATprove

In GNATprove mode the assertion policy is now always enabled, even when
analysing internal units. Otherwise, assertion expressions (e.g.
Default_Initial_Condition) in internal units (e.g. Ada.Text_IO)
disappear in the semantic analysis phase of the frontend and the
GNATprove backend can't see them.

No frontend test provided, because only the GNATprove backend is
affected (and there appear to be no difference in the output with -gnatG
switch, because the expansion of Default_Initial_Condition is not
attached to the AST).

2019-07-04  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* opt.adb (Set_Config_Switches): Keep assertions policy as
	enabled when analysing internal units in GNATprove mode.

From-SVN: r273048