Move write_graphviz_results from results.rs to graphviz.rs.

It's more graphviz-y than it is results-y. This lets us reduce
visibility of several types in `graphviz.rs`.

compiler/rustc_mir_dataflow/src/framework/mod.rs

@@ -42,7 +42,7 @@ use rustc_middle::mir::{self, BasicBlock, CallReturnPlaces, Location, Terminator
 use rustc_middle::ty::TyCtxt;
 use tracing::error;
 
-use self::results::write_graphviz_results;
+use self::graphviz::write_graphviz_results;
 use super::fmt::DebugWithContext;
 
 mod cursor;