[Ada] Restore hiding of predefined "=" operator through class-wide type

The previous change introduced a backward incompatibility in the handling
of a user-defined "=" operator for a class-wide type of a tagged type: it
would previously hide the predefined "=" operator of the tagged type in
the private case, but it no longer does in this case, while it still does
in the nonprivate case.

This hiding is a non-portability, but is fundamentally what the compiler
implements, instead of the RM rule which requires homographs.  The reason
lies in the implementation of the "=" operator in GNAT: internally, there
is not a "=" predefined operator for every nonlimited type, but instead
there is a single, universal "=" predefined operator for all the nonlimited
types.  The consequence is that the hiding rule implemented in GNAT for "="
is effectively that a user-declared symmetrical "=" operator returning
boolean hides the predefined "=" operator of any type that is covered by
the user-declared operator.

Whether it is desirable to implement the exact RM rule in GNAT is to be
discussed, but existing code relies on the non-portability and would thus
need to be changed.

gcc/ada/

	* sem_ch6.adb (New_Overloaded_Entity): Deal specifically with the
	overriding of the "=" operator for tagged types.

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