[clang][ASTImporter] Not using consumeError at failed import of in-class initializer.

The error can be returned from the function, the problem written in comment before
does not exist. The same is done already in ASTImporter at various import failures.

After a declaration is created in an `ASTNodeImporter` import function
with `GetImportedOrCreateDecl`, that function registers it with
`MapImported`. At many places import errors can happen after this
and the error is returned. The same can be done in the place where
the in-class initializer is imported.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D122528