[Parse] Split prefix operators from regex in the lexer
Teach the lexer not to consider `/` an operator
character when attempting to re-lex a regex
literal. This allows us to split off a prefix
operator.
Previously this was done after-the-fact in the
parser, but that didn't cover the unapplied infix
operator case, and didn't form a `tok::amp_prefix`
for `foo(&/.../)`, which led to a suboptimal
diagnostic.
This also now means we'll split an operator for
cases such as `foo(!/^/)` rather than treating it
as an unapplied infix operator.
rdar://92469917
