[cxx-interop] Refactor: add member operator instantiations to the lookup table elsewhere

NFC intended

Cherrypick commit 0b69eb5
Cherrypick PR #69991

Author: hjyamauchi

lib/ClangImporter/ImportDecl.cpp

@@ -2337,9 +2337,6 @@ namespace {
 
               // Make sure the synthesized decl can be found by lookupDirect.
               result->addMemberToLookupTable(opFuncDecl);
-
-              addEntryToLookupTable(*Impl.findLookupTable(decl), cxxMethod,
-                                    Impl.getNameImporter());
             }
           }
           methods.push_back(MD);
@@ -3002,6 +2999,21 @@ namespace {
       if (isSpecializationDepthGreaterThan(def, 8))
         return nullptr;
 
+      // For class template instantiations, we need to add their member
+      // operators to the lookup table to make them discoverable with
+      // unqualified lookup. This makes it possible to implement a Swift
+      // protocol requirement with an instantiation of a C++ member operator.
+      // This cannot be done when building the lookup table,
+      // because templates are instantiated lazily.
+      for (auto member : def->decls()) {
+        if (auto method = dyn_cast<clang::CXXMethodDecl>(member)) {
+          if (method->isOverloadedOperator()) {
+            addEntryToLookupTable(*Impl.findLookupTable(decl), method,
+                                  Impl.getNameImporter());
+          }
+        }
+      }
+
       return VisitCXXRecordDecl(def);
     }