Create auxiliary symbols with a different name than symbols in namespace [blocks: #4885]

[danpoe]

This adds another version of get_fresh_aux_symbol which takes a namespace and
produces symbol names that are different from any of the symbols in the
namespace. This can be used to e.g. generate fresh symbols during symex that
don't clash with any of the existing symbols.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 1e96324).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/119627321