Do not lint `dyn` tokens under macros.

The existing `KeywordIdents` lint blindly scans the token stream for a
macro or macro definition. It does not attempt to parse the input,
which means it cannot distinguish between occurrences of `dyn` that
are truly instances of it as an identifier (e.g. `let dyn = 3;`)
versus occurrences that follow its usage as a contextual keyword (e.g.
the type `Box<dyn Trait>`).

In an ideal world the lint would parse the token stream in order to
distinguish such occurrences; but in general we cannot do this,
because a macro_rules definition does not specify what parsing
contexts the macro being defined is allowed to be used within.

So rather than put a lot of work into attempting to come up with a
more precise but still incomplete solution, I am just taking the short
cut of not linting any instance of `dyn` under a macro. This prevents
`rustfix` from injecting bugs into legal 2015 edition code.

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