This is a concrete class that stores the same type of information as the IMarkers
used by the IDE. This class exists as an optimization. The various get* methods
on IMarker are extremely slow, which makes it very slow to sort markers (for example,
in the problems view). This marker class stores the fields in the most efficient form
for sorting and display, but necessarily removes some generality from IMarker.
Clears any cached information. This frees up some memory, but will slow down
the next comparison operation. It is a good idea to call this on a set of markers
after sorting them, in order to reduce their memory cost.
