[AST] Turn the callbacks of lookupInBases and forallBases into a function_ref

This lets us pass functors (and lambdas) without void * tricks. On the
downside we can't pass CXXRecordDecl's Find* members (which are now type
safe) to lookupInBases directly, but a lambda trampoline is a small
price to pay. No functionality change intended.

llvm-svn: 243217