[SR-4978] Fix compiler crash for malformed ASTs

Author: contraultra

lib/FrontendTool/ReferenceDependencies.cpp

@@ -256,7 +256,8 @@ bool swift::emitReferenceDependencies(DiagnosticEngine &diags,
     case DeclKind::Destructor:
     case DeclKind::EnumElement:
     case DeclKind::MissingMember:
-      llvm_unreachable("cannot appear at the top level of a file");
+      // These can occur in malformed ASTs.
+      break;
     }
   }
 

test/NameBinding/reference-dependencies-errors.swift

@@ -2,4 +2,8 @@
 // RUN: cp %s %t/main.swift
 // RUN: not %target-swift-frontend -typecheck -primary-file %t/main.swift -emit-reference-dependencies-path - > %t.swiftdeps
 
+associatedtype Baz
+case bar
+deinit {}
 extension Foo {}
+init() {}