The "in-class initializer" expression should be set in the field of a
default initialization expression before this expression node is created.
The `CXXDefaultInitExpr` objects are created after the AST is loaded and
at import not present in the "To" AST. And the in-class initializers of
the used fields can be missing too, these must be set at import.
This fixes a github issue #54061.
Reviewed By: martong
Differential Revision: https://reviews.llvm.org/D120824
Balázs Kéri