Revise and generalize the macros-unlinted tests.

Review feedback asked for the test to be generalized to include macros
2.0; that generalization is dyn-2015-idents-in-decl-macros-unlinted.rs

As a drive-by, I also decided to revise the test to make it clear
*why* we cannot generally lint these cases. (I already had similar
demonstrations in dyn-2015-edition-keyword-ident-lint.rs, but it does
not hurt to try to emphasize matters.)

I also added some commentary on the cases where we could choose to
make the lint smarter, namely the situations where a macro is
*definitely* using `dyn` as an identifier (because it is using it as a
path component).

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