Also fixed monospace font for d3-graphviz engine

VS code graphviz extensions use d3-graphviz, which supports `Courier`
fontname but does not support `monospace`. This caused graphs to render
poorly because the text sizes were wrong.

Author: Rich Kadel

compiler/rustc_graphviz/src/lib.rs

@@ -634,7 +634,7 @@ where
     let mut graph_attrs = Vec::new();
     let mut content_attrs = Vec::new();
     if options.contains(&RenderOption::Monospace) {
-        let font = r#"fontname="monospace""#;
+        let font = r#"fontname="Courier, monospace""#;
         graph_attrs.push(font);
         content_attrs.push(font);
     };

compiler/rustc_mir/src/util/graphviz.rs

@@ -55,7 +55,7 @@ where
     writeln!(w, "{} {}Mir_{} {{", kind, cluster, def_name)?;
 
     // Global graph properties
-    let font = r#"fontname="monospace""#;
+    let font = r#"fontname="Courier, monospace""#;
     let mut graph_attrs = vec![font];
     let mut content_attrs = vec![font];
 

src/test/mir-opt/graphviz.main.mir_map.0.dot

@@ -1,7 +1,7 @@
 digraph Mir_0_3 {
-    graph [fontname="monospace"];
-    node [fontname="monospace"];
-    edge [fontname="monospace"];
+    graph [fontname="Courier, monospace"];
+    node [fontname="Courier, monospace"];
+    edge [fontname="Courier, monospace"];
     label=<fn main() -&gt; ()<br align="left"/>>;
     bb0__0_3 [shape="none", label=<<table border="0" cellborder="1" cellspacing="0"><tr><td bgcolor="gray" align="center" colspan="1">0</td></tr><tr><td align="left" balign="left">_0 = const ()<br/></td></tr><tr><td align="left">goto</td></tr></table>>];
     bb1__0_3 [shape="none", label=<<table border="0" cellborder="1" cellspacing="0"><tr><td bgcolor="gray" align="center" colspan="1">1</td></tr><tr><td align="left">resume</td></tr></table>>];