Ada: Fix assertion failure for unfrozen mutably tagged type as actual

This is a follow-up to the fix done last December at:
  https://gcc.gnu.org/pipermail/gcc-patches/2025-December/704521.html

The fix is not sufficient when nested layers of generics are involved,
so this changes the implementation of mutably tagged types to have the
declaration of their CW-equivalent type immediately analyzed.  But this
in turn requires making sure that the root type is frozen before the
CW-equivalent type in all cases.

gcc/ada/
	PR ada/123306
	* freeze.adb (Freeze_Entity): For a class-wide equivalent type of
	a non-interface root type, freeze the root type before it.
	* sem_ch3.adb (Derived_Type_Declaration): Minor tweak.
	* sem_ch12.adb (Analyze_One_Association): Revert latest change.
	* sem_ch13.adb (Analyze_Attribute_Definition_Clause) <Size>: When
	the prefix is a class-wide type, insert the declaration of the CW-
	equivalent type immediately after that of the root type, and the
	size check for the root type into its own freezing actions.

gcc/testsuite/
	* gnat.dg/specs/mutably_tagged2.ads: New test.