Source location resolution previously made ZIR printing incredibly slow,
since it was O(N^2). Since we usually resolve source locations
approximately in order, it is much more efficient to resolve them using
a "cursor" which navigates the file.

This takes the time for `zig ast-check -t Sema.zig` down from many
minutes (enough that I got bored and killed the process; well over 10)
to a few seconds.