Support raw identifiers (backtick-delimited identifiers containing non-identifier characters).

[allevato]

Do not merge—this is for the raw identifiers pitch.

This PR still has some failing tests where imported C++ template symbols are referenced in printed SIL. It looks like the identifier itself contains the full generic signature including <, >, and other punctuation, and this causes the "must escape" logic in SILPrinter to put backticks around them. I'm not sure if that's correct (i.e., will that round-trip if the SIL is parsed back in).