ada: Early freezeing of types with 'Size'Class

ada: Early freezeing of types with 'Size'Class

This patch fixes an issue in the compiler whereby declarations of derived types
whose parent is a mutably tagged type cause early freezing of the parent type -
leading to spurious compile-time errors.

gcc/ada/ChangeLog:
	* sem_ch3.adb (Derived_Type_Declaration): Modify generation of
	compile time check.