sess: stabilize --terminal-width

Formerly `-Zterminal-width`, `--terminal-width` allows the user or build
tool to inform rustc of the width of the terminal so that diagnostics
can be truncated.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_interface/src/tests.rs

@@ -689,7 +689,6 @@ fn test_debugging_options_tracking_hash() {
     untracked!(span_debug, true);
     untracked!(span_free_formats, true);
     untracked!(temps_dir, Some(String::from("abc")));
-    untracked!(terminal_width, Some(80));
     untracked!(threads, 99);
     untracked!(time, true);
     untracked!(time_llvm_passes, true);