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_parse/src/parser/expr.rs

@@ -1917,7 +1917,6 @@ impl<'a> Parser<'a> {
                         BREAK_WITH_LABEL_AND_LOOP,
                         lo.to(expr.span),
                         ast::CRATE_NODE_ID,
-                        "this labeled break expression is easy to confuse with an unlabeled break with a labeled value expression",
                         BuiltinLintDiag::BreakWithLabelAndLoop(expr.span),
                     );
                 }