errors: lazily load fallback fluent bundle

Loading the fallback bundle in compilation sessions that won't go on to
emit any errors unnecessarily degrades compile time performance, so
lazily create the Fluent bundle when it is first required.

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

compiler/rustc_errors/src/lib.rs

@@ -33,7 +33,7 @@ use rustc_data_structures::sync::{self, Lock, Lrc};
 use rustc_data_structures::AtomicRef;
 pub use rustc_error_messages::{
     fallback_fluent_bundle, fluent_bundle, DiagnosticMessage, FluentBundle, LanguageIdentifier,
-    MultiSpan, SpanLabel,
+    LazyFallbackBundle, MultiSpan, SpanLabel, DEFAULT_LOCALE_RESOURCES,
 };
 pub use rustc_lint_defs::{pluralize, Applicability};
 use rustc_serialize::json::Json;
@@ -547,7 +547,7 @@ impl Handler {
         treat_err_as_bug: Option<NonZeroUsize>,
         sm: Option<Lrc<SourceMap>>,
         fluent_bundle: Option<Lrc<FluentBundle>>,
-        fallback_bundle: Lrc<FluentBundle>,
+        fallback_bundle: LazyFallbackBundle,
     ) -> Self {
         Self::with_tty_emitter_and_flags(
             color_config,
@@ -562,7 +562,7 @@ impl Handler {
         color_config: ColorConfig,
         sm: Option<Lrc<SourceMap>>,
         fluent_bundle: Option<Lrc<FluentBundle>>,
-        fallback_bundle: Lrc<FluentBundle>,
+        fallback_bundle: LazyFallbackBundle,
         flags: HandlerFlags,
     ) -> Self {
         let emitter = Box::new(EmitterWriter::stderr(