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.
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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.