ada: Fix detection of external calls to protected objects in instances

ada: Fix detection of external calls to protected objects in instances

Detection of external-vs-internal calls to protected objects relied on
the scope stack. This didn't work when the call appeared in an instance
of generic unit, because instances are analyzed in different context to
where they appear.

gcc/ada/

	* exp_ch6.adb (Expand_Protected_Subprogram_Call): Examine scope
	tree and not the scope stack.