[analyzer][NFC] Remove useless class BuiltinBug

...because it provides no useful functionality compared to its base
class `BugType`.

A long time ago there were substantial differences between `BugType` and
`BuiltinBug`, but they were eliminated by commit 1bd58233 in 2009 (!).
Since then the only functionality provided by `BuiltinBug` was that it
specified `categories::LogicError` as the bug category and it stored an
extra data member `desc`.

This commit sets `categories::LogicError` as the default value of the
third argument (bug category) in the constructors of BugType and
replaces use of the `desc` field with simpler logic.

Note that `BugType` has a data member `Description` and a non-virtual
method `BugType::getDescription()` which queries it; these are distinct
from the member `desc` of `BuiltinBug` and the identically named method
`BuiltinBug::getDescription()` which queries it. This confusing name
collision was a major motivation for the elimination of `BuiltinBug`.

As this commit touches many files, I avoided functional changes and left
behind FIXME notes to mark minor issues that should be fixed later.

Differential Revision: https://reviews.llvm.org/D158855