Store goto_symex::\guard in a single place [blocks: #3619]

[tautschnig]

The static member of a static function will ensure both proper initialisation
and making it available to all users.

This is factored out from #3619 to make it easier to pinpoint what exactly causes trouble with TG. The commit conflict-resolved version of the third one from #3619, which has been approved already.

• Each commit message has a non-empty body, explaining why the change was made.
• 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.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 7562b09).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99328913

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 9eb1c9b).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99414751
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

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 incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[tautschnig]

@romainbrenguier confirmed that this passes TG tests.

[kroening]

symex_targett seems an odd place for that method? It's a container for the output of symex.

[peterschrammel]

symex_targett seems an odd place for that method? It's a container for the output of symex.

@tautschnig, maybe in a separate file?

[tautschnig]

symex_targett seems an odd place for that method? It's a container for the output of symex.

I should be able to put it in goto_symex_statet I guess. I'll give that a try.

[tautschnig]

I have moved the declaration to goto_symex_statet.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 58bd9ed).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99574553