[analyzer] Catch the first taint propagation implied buffer overflow.

Change the ArrayBoundCheckerV2 to be more aggressive in reporting buffer overflows
when the offset is tainted. Previously, we did not report bugs when the state was
underconstrained (not enough information about the bound to determine if there is
an overflow) to avoid false positives. However, if we know that the buffer
offset is tainted - comes in from the user space and can be anything, we should
report it as a bug.

+ The very first example of us catching a taint related bug.
This is the only example we can currently handle. More to come...

llvm-svn: 144826