Fix/suppress misc phan errors (#2)

Fix/suppress misc phan errors (#2)

* Title: phan false positive
* McrUndoAction: fixed improper use of @param
* UploadSourceAdapter: fixed wrong type
* XmlTypeCheck: Use null so phan doesn't think we're trying to call the
function ''
* Database: phan false positive
* SpecialBlock: Use phan's advanced type documentation so phan knows
specifically what's being returned
* ChangesListSpecialPage: phan false positive
* BatchRowUpdate: Have default callback take a parameter so phan doesn't
think too many arguments are being passed
* MimeAnalyzer: left FIXME for relying on PHP 7.1 unpack() signature
* LanguageConverter: Specify types for $mTables since phan couldn't
determine it automatically
* preprocessorFuzzTest: Implement User::load() method signature

Change-Id: I08080ab636c5fe67ea6a4e14b2212d7523606e21