[compiler] Simplify FunctionExpression node

Rather than storing the entire babel node, store only the required
information which is the node type.

This will be useful for when we synthesize new functions that don't have
a corresponding babel node.

ghstack-source-id: 9098cbdbc4b1e9a6e7dafa2e7645f6f4854e1eac
Pull Request resolved: https://github.com/facebook/react/pull/30544