struct_typet::baset: no need for cast to symbol_type

[kroening]

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• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[allredj]

Passed Diffblue compatibility checks (cbmc commit: c134980).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86472408