Generate lint diagnostic message from BuiltinLintDiag

Translation of the lint message happens when the actual diagnostic is
created, not when the lint is buffered. Generating the message from
BuiltinLintDiag ensures that all required data to construct the message
is preserved in the LintBuffer, eventually allowing the messages to be
moved to fluent.

Remove the `msg` field from BufferedEarlyLint, it is either generated
from the data in the BuiltinLintDiag or stored inside
BuiltinLintDiag::Normal.

compiler/rustc_lint/src/context.rs

@@ -539,12 +539,11 @@ pub trait LintContext {
         &self,
         lint: &'static Lint,
         span: Option<impl Into<MultiSpan>>,
-        msg: impl Into<DiagMessage>,
         decorate: impl for<'a, 'b> FnOnce(&'b mut Diag<'a, ()>),
         diagnostic: BuiltinLintDiag,
     ) {
         // We first generate a blank diagnostic.
-        self.opt_span_lint(lint, span, msg, |db| {
+        self.opt_span_lint(lint, span, diagnostics::builtin_message(&diagnostic), |db| {
             // Now, set up surrounding context.
             diagnostics::builtin(self.sess(), diagnostic, db);
             // Rewrap `db`, and pass control to the user.