Add -Zgraphviz_dark_mode

Many developers use a dark theme with editors and IDEs, but this
typically doesn't extend to graphviz output.

When I bring up a MIR graphviz document, the white background is
strikingly bright. This new option changes the colors used for graphviz
output to work better in dark-themed UIs.

compiler/rustc_session/src/options.rs

@@ -907,6 +907,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "force all crates to be `rustc_private` unstable (default: no)"),
     fuel: Option<(String, u64)> = (None, parse_optimization_fuel, [TRACKED],
         "set the optimization fuel quota for a crate"),
+    graphviz_dark_mode: bool = (false, parse_bool, [UNTRACKED],
+        "use dark-themed colors in graphviz output (default: no)"),
     hir_stats: bool = (false, parse_bool, [UNTRACKED],
         "print some statistics about AST and HIR (default: no)"),
     human_readable_cgu_names: bool = (false, parse_bool, [TRACKED],