Don't invent a '$auto-x-y' name for auto types in generic lambdas. This is no

better than the 'template-parameter-x-y' name that we'd get in AST printing,
and is worse in several ways (it's harder to distinguish it from a
user-supplied name, it's wrong after substituting some number of outer
levels, it wastes time and space constructing an IdentifierInfo, ...).

llvm-svn: 225489