[pseudo] Only expand UCNs for raw_identifiers

It turns out clang::expandUCNs only works on tokens that contain valid UCNs
and no other random escapes, and clang only uses it on raw_identifiers.

Currently we can hit an assertion by creating tokens with stray non-valid-UCN
backslashes in them.

Fortunately, expanding UCNs in raw_identifiers is actually all we need.
Most tokens (keywords, punctuation) can't have them. UCNs in literals can be
treated as escape sequences like \n even this isn't the standard's
interpretation. This more or less matches how clang works.
(See https://isocpp.org/files/papers/P2194R0.pdf which points out that the
standard's description of how UCNs work is misaligned with real implementations)

Differential Revision: https://reviews.llvm.org/D125049