Add an identifier-based lookup to constant_propagator_is_constantt [blocks: #3768]

[tautschnig]

This avoid the workaround of constructing a symbol_exprt without proper type
just for the sake of a lookup.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 4455fb5).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98051548