Support multi-character punct tokens in MBE
Fixes#11497
In the context of MBE, consecutive puncts are parsed as multi-character punct tokens whenever possible. For example, `:::` is parsed as ``[Punct(`::`), Punct(`:`)]`` and shouldn't get matched to patterns like `: : :` or `: ::`.
We have implemented this behavior only for when we match puncts against `tt` fragments, but not when we match puncts literally. This PR extracts the multi-character punct handling procedure into a separate method and extends its support for literal matching.
For good measure, this PR adds support for `<-` token, which is still [considered as one token in rustc](e396186407/compiler/rustc_ast/src/token.rs (L249)) despite the placement syntax having been removed.
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