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_codegen_llvm/src/back/write.rs

@@ -417,7 +417,7 @@ fn report_inline_asm(
     }
     let level = match level {
         llvm::DiagnosticLevel::Error => Level::Error,
-        llvm::DiagnosticLevel::Warning => Level::Warning(None),
+        llvm::DiagnosticLevel::Warning => Level::Warning,
         llvm::DiagnosticLevel::Note | llvm::DiagnosticLevel::Remark => Level::Note,
     };
     cgcx.diag_emitter.inline_asm_error(cookie as u32, msg, level, source);