Catching whitespace and comments is no problem in Antlr[1]. I'm not exactly sure what you mean by round-tripping. In terms of how daunting it is, it's much easier than trying to use Flex/Bison or lex/yacc, that's for sure.

1: At the bottom of the grammar, there are rules to intentionally skip over whitespace and comments, but it could have just as easily captured them and parsed them further. https://github.com/antlr/grammars-v4/blob/master/c/C.g4