Regression tests: Ensure __LINE__ is expanded

[tautschnig]

We previously generated several declarations with the same name, because
the C preprocessor does not expand LINE after a concatenation
operator. Add two levels of indirection to achieve this. Using
declarations with the same name was kind of ok for the C front-end, but
is a bug with any C++ front-end.

This is in preparation of running a subset of the C test against the C++ front-end.

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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 949b116).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108999105
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.

[kroening]

We could contemplate _Static_assert!

[tautschnig]

We could contemplate _Static_assert!

Hmm, yes, but then we'd also have to add -std=c11 to all the goto-cc invocations?