[analyzer] Reduce code duplication: make CXXDestructorCall a CXXInstanceCall.

While there is now some duplication between SimpleCall and the CXXInstanceCall
sub-hierarchy, this is much better than copy-and-pasting the devirtualization
logic shared by both instance methods and destructors.

An unfortunate side effect is that there is no longer a single CallEvent type
that corresponds to "calls written as CallExprs". For the most part this is a
good thing, but the checker callback eval::Call still takes a CallExpr rather
than a CallEvent (since we're not sure if we want to allow checkers to
evaluate other kinds of calls). A mistake here will be caught by a cast<> in
CheckerManager::runCheckersForEvalCall.

No functionality change.

llvm-svn: 161809