[AST] [Modules] Handle full cases of DefaultArgStorage::setInherited

There were two assertions in DefaultArgStorage::setInherited previously.
It requires the DefaultArgument is either empty or an ar

[AST] [Modules] Handle full cases of DefaultArgStorage::setInherited

There were two assertions in DefaultArgStorage::setInherited previously.
It requires the DefaultArgument is either empty or an argument value. It
would crash if it has a pointer refers to the previous declaration or
contains a chain to the previous declaration.

But there are edge cases could hit them actually. One is
InheritDefaultArguments.cppm that I found recently. Another one is pr31469.cpp,
which was created fives years ago.

This patch tries to fix the two failures by handling full cases in
DefaultArgStorage::setInherited.

This is guaranteed to not introduce any breaking change since it lives
in the path we wouldn't touch before. And the added assertions for
sameness should keep the correctness.

Reviewed By: v.g.vassilev

Differential Revision: https://reviews.llvm.org/D128974

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