[Lex] Don't assert when decoding invalid UCNs.

Currently if a lexically-valid UCN encodes an invalid codepoint, then we
diagnose that, and then hit an assertion while trying to decode it.

Since there isn't anything preventing us reaching this state, remove the
assertion. expandUCNs("X\UAAAAAAAAY") will produce "XY".

Differential Revision: https://reviews.llvm.org/D125059