Sorry, just wanted to save your time.
There is a difference between AsmParser::Lex() and AsmLexer::Lex(). The latter is low-level function that should rarely be called in target’s AsmParser, if at all.
The issue will go away if you replace getLexer().Lex() with just Lex() or getParser().Lex().
There are also two typos in your example:
/* this is NOT fine * /0x1234/
→
/* this is NOT fine */0x1234