ada: Derived type with convention C must override convention C_Pass_By_Copy

ada: Derived type with convention C must override convention C_Pass_By_Copy

If a type DT is derived from a record type T with convention C_Pass_By_Copy
and explicitly specifies convention C (via aspect or pragma), then type DT
should not be treated as a type with convention C_Pass_By_Copy. Any parameters
of the derived type should be passed by reference rather than by copy. The
compiler was incorrectly inheriting convention C_Pass_By_Copy, by inheriting
the flag set on the parent type, but that flag needs to be unset in the case
where the convention is overridden.

gcc/ada/

	* sem_prag.adb (Set_Convention_From_Pragma): If the specified convention on
	a record type is not C_Pass_By_Copy, then force the C_Pass_By_Copy flag to
	False, to ensure that it's overridden.