Try to lex identifiers before punctuation, and backtrack if we only
see a single underscore or something that starts with two underscores.
Rename the modf and frexp return types to start with two underscores.
Spec PR:
https://github.com/gpuweb/gpuweb/pull/2326
Change-Id: Id283af100babfe84faa183345cb8a60848140caa
Fixed: tint:1292
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/70160
Reviewed-by: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
James Price