preprocessor: line-map cleanups

preprocessor: line-map cleanups

I found the linemap logic dealing with running out of column numbers
confusing.  There's no need for completely separate code blocks there,
as we can rely on the masking operations working all the way down to
zero bits.  The two binary searches for linemap lookups could do with
modernization of placing the var decls at their initialization point.
(These two searches work in opposite directions, and while lower_bound
would work there, the caching got in the way and I decided to be
conservative.)

	libcpp/
	* line-map.c (linemap_add): Simplify column overflow calculation.
	Add comment about range and column bit init.
	(linemap_ordinary_map_lookup): Refactor for RAII
	(linemap_macro_map_lookup): Likewise.