[Metadata reader] Save/restore symbolic reference resolver

… rather than clearing it out completely. This is better hygiene, although it
doesn’t matter right now.

Author: DougGregor

include/swift/Demangling/Demangler.h

@@ -507,7 +507,13 @@ class Demangler : public NodeFactory {
                           std::function<SymbolicReferenceResolver_t> resolver) {
     SymbolicReferenceResolver = resolver;
   }
-  
+
+  /// Take the symbolic reference resolver.
+  std::function<SymbolicReferenceResolver_t> &&
+  takeSymbolicReferenceResolver() {
+    return std::move(SymbolicReferenceResolver);
+  }
+
   /// Demangle the given symbol and return the parse tree.
   ///
   /// \param MangledName The mangled symbol string, which start with the

include/swift/Remote/MetadataReader.h

@@ -1521,6 +1521,7 @@ class MetadataReader {
     }
 
     // Install our own symbolic reference resolver
+    auto oldSymbolicReferenceResolver = dem.takeSymbolicReferenceResolver();
     dem.setSymbolicReferenceResolver([&](SymbolicReferenceKind kind,
                                          Directness directness,
                                          int32_t offset,
@@ -1562,7 +1563,7 @@ class MetadataReader {
       break;
     }
 
-    dem.setSymbolicReferenceResolver(nullptr);
+    dem.setSymbolicReferenceResolver(std::move(oldSymbolicReferenceResolver));
     return result;
   }