Rollup merge of #81856 - Smittyvb:utf16-warn, r=matthewjasper

Rollup merge of #81856 - Smittyvb:utf16-warn, r=matthewjasper

Suggest character encoding is incorrect when encountering random null bytes

This adds a note whenever null bytes are seen at the start of a token unexpectedly, since those tend to come from UTF-16 encoded files without a [BOM](https://en.wikipedia.org/wiki/Byte_order_mark) (if a UTF-16 BOM appears it won't be valid UTF-8, but if there is no BOM it be both valid UTF-16 and valid but garbled UTF-8). This approach was suggested in https://github.com/rust-lang/rust/issues/73979#issuecomment-653976451.

Closes #73979.