GenCode, Cache::cleanKey() - Fix deploop during clean initialization

GenCode, Cache::cleanKey() - Fix deploop during clean initialization

Overview
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Suppose you have a clean codebase with no DAO files, and you try to run `xml/GenCode.php`.
This currently crashes.

The problem is not symptomatic day-to-day because we commit DAOs, so it's
very rare to run a full/clean initialization.  However, it does get in the
way of stress-testing the correctness of GenCode.

Before
------

Since 76e697a9d750d568d0d12c10dd8173f656e8bb1e, I believe we've had a dependency-loop
in the clean-gencode scenario, which works as follows:

* We don't have any DAOs, so we run `GenCode`.
* Running `GenCode` does a partial bootstrap (i.e. `CRM_Core_Config::singleton($loadFromDB===FALSE)`)
* The partial bootstrap creates some thread-local caches (`Arraycache`)
* Before creating the cache, it normalizes the name by calling `CRM_Core_BAO_Cache::cleanKey()`
* `CRM_Core_BAO_Cache` extends `CRM_Core_DAO_Cache`
* `CRM_Core_BAO_Cache` doesn't exist - that's why we called `GenCode` at the beginning.

After
-----

This migrates the utility function `cleanKey()` to another class which is always available and
does not rely on DAOs.