Previously we would strictly match `{` + `}`, but
that ignored the fact that when parsing we consider
`#if` + `#endif` to be a stronger delimiter than
`{` + `}`, so can ignore a stray `}` in a `#if`.
Update the logic to also track opening and closing
`#if` decls, ignoring any braces that happen within
them.
rdar://129195380
