compiler: fix some clippy needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_session\src\config.rs:2111:20
     |
2111 |     unstable_opts: &mut UnstableOptions,
     |                    ^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&UnstableOptions`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

compiler/rustc_session/src/config.rs

@@ -2108,7 +2108,7 @@ fn should_override_cgus_and_disable_thinlto(
 fn collect_print_requests(
     early_dcx: &EarlyDiagCtxt,
     cg: &mut CodegenOptions,
-    unstable_opts: &mut UnstableOptions,
+    unstable_opts: &UnstableOptions,
     matches: &getopts::Matches,
 ) -> Vec<PrintRequest> {
     let mut prints = Vec::<PrintRequest>::new();
@@ -2732,6 +2732,8 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
     }
 
     if let Ok(graphviz_font) = std::env::var("RUSTC_GRAPHVIZ_FONT") {
+        // FIXME: this is only mutation of UnstableOptions here, move into
+        // UnstableOptions::build?
         unstable_opts.graphviz_font = graphviz_font;
     }
 
@@ -2781,7 +2783,7 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
         ));
     }
 
-    let prints = collect_print_requests(early_dcx, &mut cg, &mut unstable_opts, matches);
+    let prints = collect_print_requests(early_dcx, &mut cg, &unstable_opts, matches);
 
     let cg = cg;