ada: Fix missing detection of late equality operator returning subtype of Boolean

ada: Fix missing detection of late equality operator returning subtype of Boolean

In Ada 2012, the compiler fails to check that a primitive equality operator
for an untagged record type must appear before the type is frozen, when the
operator returns a subtype of Boolean.  This plugs the legality loophole but
adds the debug switch -gnatd_q to go back to the previous state.

gcc/ada/ChangeLog:

	PR ada/18765
	* debug.adb (d_q): Document new usage.
	* sem_ch6.adb (New_Overloaded_Entity): Apply the special processing
	to all equality operators whose base result type is Boolean, but do
	not enforce the new Ada 2012 freezing rule if the result type is a
	proper subtype of it and the -gnatd_q switch is specified.