[lldb][ASTUtils] Short-circuit multiplexed CompleteRedeclChain when we find definition

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h

@@ -281,9 +281,16 @@ class SemaSourceWithPriorities : public ImporterBackedASTSource {
     return nullptr;
   }
 
+  /// Call ExternalASTSource::CompleteRedeclChain(D)
+  /// on each AST source. Returns as soon as we got
+  /// a definition for D.
   void CompleteRedeclChain(const clang::Decl *D) override {
-    for (size_t i = 0; i < Sources.size(); ++i)
+    for (size_t i = 0; i < Sources.size(); ++i) {
       Sources[i]->CompleteRedeclChain(D);
+      if (auto *td = llvm::dyn_cast<clang::TagDecl>(D))
+        if (td->getDefinition())
+          return;
+    }
   }
 
   clang::Selector GetExternalSelector(uint32_t ID) override {