[analyzer] Remove impossible BugType from CStringChecker (#152163)

CStringChecker had an AdditionOverflow bug type which was intended for a
situation where the analyzer concludes that the addition of two
size/length values overflows `size_t`.

I strongly suspect that the analyzer could emit bugs of this type in
certain complex corner cases (e.g. due to inaccurate cast modeling), but
these reports would be all false positives because in the real world the
sum of two size/length values is always far below SIZE_MAX. (Although
note that there was no test where the analyzer emitted a bug with this
type.)

To simplify the code (and perhaps eliminate false positives), I
eliminated this bug type and replaced code that emits it by a simple
`addSink()` call (because we still want to get rid of the execution
paths where the analyzer has invalid assumptions).

0 files changed, 0 insertions, 0 deletions