It's silently converted to just "x.com", though I'm not sure what standard that's based on. It's not an IDN as the other comment suggests, as that character is disallowed under IDNA2008.
It doesn't show up in a couple of DNS checkers that I tried, and I can't find a punycode converter that does anything beyond spit out a plain X. So my guess is that something in the browser normalizes it before it ever gets resolved by DNS.
[0] https://en.wikipedia.org/wiki/Homoglyph
[1] https://en.wikipedia.org/wiki/IDN_homograph_attack