lint for: unsafe_unicode

[pq]

Context:

• https://krebsonsecurity.com/2021/11/trojan-source-bug-threatens-the-security-of-all-code/

[pq]

Interestingly Rust has broken this into two lints (for comments and literals):

https://doc.rust-lang.org/beta/rustc/lints/listing/deny-by-default.html#text-direction-codepoint-in-comment

I'm not sure we need to do that but I do think it's worth considering if we should go through the trouble (and expense) of flagging the enclosing comment or literal rather than just the start of the file.

/fyi @mit-mit

[pq]

It looks like we're moving towards implementing this as a hint; see: #47611.

[pq]

Closing in favor of tracking on the SDK issue (#47611).