[Ada] Fix spurious error on derived private type with predicate

[Pierre-Marie de Rodat]

As explained in the head comment of Compatible_Types_In_Predicate,
anomalies involving private and full views can occur when a call
to a predicate or invariant function is generated by the compiler.

The function uses the child function Common_Type to reconcile the
various views of a private type, but this latter function does not
consider the Underlying_Full_View of private types, so it can miss
some cases like this one.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-16  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch4.adb (Common_Type): Go to Underlying_Full_View, if any.
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