[analyzer] Replace -analyzer-no-eagerly-trim-egraph with graph-trim-interval.

After every 1000 CFGElements processed, the ExplodedGraph trims out nodes
that satisfy a number of criteria for being "boring" (single predecessor,
single successor, and more). Rather than controlling this with a cc1 option,
which can only disable this behavior, we now have an analyzer-config option,
'graph-trim-interval', which can change this interval from 1000 to something
else. Setting the value to 0 disables reclamation.

The next commit relies on this behavior to actually test anything.

llvm-svn: 166528