This commit generalizes the existing `async_idents` lint to easily encompass
other identifiers that will be keywords in future editions. The new lint is
called `keyword_idents` and the old `async_idents` lint is registered as renamed
to this new lint.
As a proof of concept the `try` keyword was added to this list as it looks to be
listed as a keyword in the 2018 edition only. The `await` keyword was not added
as it's not listed as a keyword yet.
Closes#53077
* Enable the `raw_identifiers` feature automatically in the 2018 preview
* Only emit lint warnings if the `raw_identifiers` feature is activated
cc rust-lang/cargo#5783
