Change how force-warn lint diagnostics are recorded.

`is_force_warn` is only possible for diagnostics with `Level::Warning`,
but it is currently stored in `Diagnostic::code`, which every diagnostic
has.

This commit:
- removes the boolean `DiagnosticId::Lint::is_force_warn` field;
- adds a `ForceWarning` variant to `Level`.

Benefits:
- The common `Level::Warning` case now has no arguments, replacing
  lots of `Warning(None)` occurrences.
- `rustc_session::lint::Level` and `rustc_errors::Level` are more
  similar, both having `ForceWarning` and `Warning`.

compiler/rustc_expand/src/proc_macro_server.rs

@@ -380,7 +380,7 @@ impl ToInternal<rustc_errors::Level> for Level {
     fn to_internal(self) -> rustc_errors::Level {
         match self {
             Level::Error => rustc_errors::Level::Error,
-            Level::Warning => rustc_errors::Level::Warning(None),
+            Level::Warning => rustc_errors::Level::Warning,
             Level::Note => rustc_errors::Level::Note,
             Level::Help => rustc_errors::Level::Help,
             _ => unreachable!("unknown proc_macro::Level variant: {:?}", self),