parse: Tweak the function parameter edition check
Follow-up to https://github.com/rust-lang/rust/pull/69801.
Edition of a code fragment is inferred from "the place where the code is written".
For individual tokens like edition-specific keywords it may be the span of the token itself ("uninterpolated" span), but for larger code fragments it's probably not, in the test example the trait method is obviously written in "2015 edition code".
r? @Centril
6a8683fcd0