Update to Boogie 2.15.9

[gauravpartha]

No description provided.

[gauravpartha]

Locally on my machine, the tests run around 15% slower in total using the updated Boogie version. It would be nice to use the new /normalizeNames:1 Boogie option (generates names in the SMT file in a way that is more robust to changes in the Boogie program). However, on my machine, the verifying test https://github.com/viperproject/silver/blob/master/src/test/resources/all/issues/silicon/0483a.vpr then goes from around 45 seconds to 6 minutes. So, we decided to not use the /normalizeNames:1 option for now. It is not clear why the verification time changes by so much for this file if one uses /normalizeNames:1; it would make sense to investigate this at some point.