Fix typet::check usage to work with latest API

[tautschnig]

Fixes the build breakage caused by merging an old PR. We nowadays require a
validation mode.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: f05d3a0).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/103114603
Status will be re-evaluated on next push.
Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)

• the author is not in the list of contributors (e.g. first-time contributors).

• the compatibility was already broken by an earlier merge.