C++ front-end: bool is also an integral type [blocks: #3659]

[tautschnig]

Visual Studio uses enums with a bool subtype in header files.

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[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 0872568).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96952525
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.