swift234

[SR-1545] Implement lexing of source conflict markers. (#2924)

Lexing for conflict markers is inspired by the way clang handles them with a few
updates of our own.  Clang's lexer currently searches for the start of the
conflict marker, attempts to find the divider points, lexes that, then finds the
end.  We, unfortunately, cannot be so forgiving because of operator overloads.
Instead, we search for the start and end markers and ignore all text in between.
Even if what is found is not conflict markers, it certainly is not valid Swift either.