C++ front-end: store typedef names

[tautschnig]

This is already supported in the C front-end, and we should not
unnecessarily deviate in behaviour between the C and C++ front-ends.

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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]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 818f339).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108850119
Status will be re-evaluated on next push.
Common spurious failures include: 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); compatibility was already broken by an earlier merge.