[ClangImporter] Deserialize only decls with a matching @objc name

Prevents a cycle where a general access to all top level decls lead the
deserialization to call ClangImporter which triggered more
deserialization. It crashed the compiler in a complex case so only
limiting what is deserialized should be enough to fix this for now.

rdar://problem/57118844

Author: xymus

lib/ClangImporter/ImportDecl.cpp

@@ -4563,17 +4563,25 @@ namespace {
 
       if (!found) {
         // Try harder to find a match looking at just custom Objective-C names.
-        SmallVector<Decl *, 64> allTopLevelDecls;
-        overlay->getTopLevelDecls(allTopLevelDecls);
-        for (auto result : allTopLevelDecls) {
+        // Limit what we deserialize to decls with an @objc attribute.
+        SmallVector<Decl *, 4> matchingTopLevelDecls;
+
+        // Get decls with a matching @objc attribute
+        overlay->getTopLevelDeclsWhereAttributesMatch(
+          matchingTopLevelDecls,
+          [&name](const DeclAttributes attrs) -> bool {
+            if (auto objcAttr = attrs.getAttribute<ObjCAttr>())
+              if (auto objcName = objcAttr->getName())
+                return objcName->getSimpleName() == name;
+            return false;
+          });
+
+        // Filter by decl kind
+        for (auto result : matchingTopLevelDecls) {
           if (auto singleResult = dyn_cast<T>(result)) {
-            // The base name _could_ match but it's irrelevant here.
-            if (isMatch(singleResult, /*baseNameMatches=*/false,
-                        /*allowObjCMismatch=*/false)) {
-              if (found)
-                return nullptr;
-              found = singleResult;
-            }
+            if (found)
+              return nullptr;
+            found = singleResult;
           }
         }
       }