Copy constructor parameter instead of storing reference

[NlightNFotis]

Previously we stored a reference to the name_prefix parameter in allocate objects that led to segfaults if it was constructed with a temporary. Now we store a copy instead, which prevents that from
happening.

This is work that we did jointly @hannes-steffenhagen-diffblue as part of the work on the goto-harness feature to fix a bug we discovered.

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[smowton]

Use normal decl syntax otherwise lgtm

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 14cfc08).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99276795