Make some marginal performance improvements in reportErrorPosition(),

Make some marginal performance improvements in reportErrorPosition(),
which turns out to be a dominant part of the runtime in scenarios
involving lots of parse-time warnings (such as Stephen Frost's example
of an INSERT with a lot of backslash-containing strings).  There's not
a whole lot we can do about the character-at-a-time scanning, but we
can at least avoid traversing the query twice.