Simplify LazyAttrTokenStream

[nnethercote]

LazyAttrTokenStream is an unpleasant type: Lrc<Box<dyn ToAttrTokenStream>>. Why does it look like that?

• There are two ToAttrTokenStream impls, one for the lazy case, and one for the case where we already have an AttrTokenStream.
• The lazy case (LazyAttrTokenStreamImpl) is implemented in rustc_parse, but LazyAttrTokenStream is defined in rustc_ast, which does not depend on rustc_parse. The use of the trait lets rustc_ast implicitly depend on rustc_parse. This explains the dyn.
• LazyAttrTokenStream must have a size_of as small as possible, because it's used in many AST nodes. This explains the Lrc<Box<_>>, which keeps it to one word. (It's required Lrc<dyn _> would be a fat pointer.)

This PR moves LazyAttrTokenStreamImpl (and a few other token stream things) from rustc_parse to rustc_ast. This lets us replace the ToAttrTokenStream trait with a two-variant enum and also remove the Box, changing LazyAttrTokenStream to Lrc<LazyAttrTokenStreamInner>. Plus it does a few cleanups.

r? @petrochenkov