Revert "Promote missing_fragment_specifier to hard error"

This reverts commit 02eae432e7.

compiler/rustc_session/src/parse.rs

@@ -119,6 +119,7 @@ pub struct ParseSess {
     pub unstable_features: UnstableFeatures,
     pub config: CrateConfig,
     pub edition: Edition,
+    pub missing_fragment_specifiers: Lock<FxHashMap<Span, NodeId>>,
     /// Places where raw identifiers were used. This is used for feature-gating raw identifiers.
     pub raw_identifier_spans: Lock<Vec<Span>>,
     /// Used to determine and report recursive module inclusions.
@@ -152,6 +153,7 @@ impl ParseSess {
             unstable_features: UnstableFeatures::from_environment(None),
             config: FxHashSet::default(),
             edition: ExpnId::root().expn_data().edition,
+            missing_fragment_specifiers: Default::default(),
             raw_identifier_spans: Lock::new(Vec::new()),
             included_mod_stack: Lock::new(vec![]),
             source_map,