Add a missing 'classof' to AST Node TypoExpr to identify its 'Kind'.

I'm not sure why TypoExpr had its classof left out - but I expect every AST node should fulfill the 'contract of classof' (http://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html).

There should be no functionality change.  I just happened to notice it was missing, while messing around with something else.

llvm-svn: 245812