J2CL: use a more future proof naming convention for once functions

[gkdn]

Existing convention uses _@once@_ but we also use @ for class separation.
It is cleaner&more future proof to use something other convention like _<once>_.

[gkdn]

@kripken renamed _@once@_ to _<once>_ since we decided to use that convention instead.

[kripken]

Looks like that didn't work. From the error I'm not sure what's wrong.

Unfortunately I don't have much experience with either windows or the lit test runner that is used here, so I don't have any good guesses aside from "likely an escaping issue related to windows". So we might need to debug on a windows machine if we can't find a workaround.

However @tlively perhaps your knowledge of lit can help here - are there best practices from LLVM for avoiding windows-specific escaping issues?

[gkdn]

@kripken I added a workaround with a note PTAL.

[tlively]

I don't have a fix for the Windows escaping off the top of my head. The first thing I would try is doubling up on the backslashes and if that doesn't work I would debug on a Windows box. This workaround lgtm, though.