skip implicit clang decls in the duplicate-decl assertion

SR-15801

rdar://88299954

Author: QuietMisdreavus

lib/AST/Module.cpp

@@ -963,6 +963,14 @@ void ModuleDecl::getDisplayDecls(SmallVectorImpl<Decl*> &Results) const {
 #ifndef NDEBUG
   llvm::DenseSet<Decl *> visited;
   for (auto *D : Results) {
+    // decls synthesized from implicit clang decls may appear multiple times;
+    // e.g. if multiple modules with underlying clang modules are re-exported.
+    // including duplicates of these is harmless, so skip them when counting
+    // this assertion
+    if (const auto *CD = D->getClangDecl()) {
+      if (CD->isImplicit()) continue;
+    }
+
     auto inserted = visited.insert(D).second;
     assert(inserted && "there should be no duplicate decls");
   }

test/SymbolGraph/ClangImporter/Submodules.swift

@@ -1,5 +1,7 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -I %S/Inputs/Submodules -emit-module-path %t/Submodules.swiftmodule -enable-objc-interop -module-name Submodules %s -emit-symbol-graph -emit-symbol-graph-dir %t
+// RUN: cp -r %S/Inputs/EmitWhileBuilding/EmitWhileBuilding.framework %t
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -enable-objc-interop -emit-module-path %t/EmitWhileBuilding.framework/Modules/EmitWhileBuilding.swiftmodule/%target-swiftmodule-name -import-underlying-module -F %t -module-name EmitWhileBuilding -disable-objc-attr-requires-foundation-module %S/EmitWhileBuilding.swift
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -I %S/Inputs/Submodules -emit-module-path %t/Submodules.swiftmodule -enable-objc-interop -module-name Submodules -F %t %s -emit-symbol-graph -emit-symbol-graph-dir %t
 
 // REQUIRES: objc_interop
 
@@ -8,4 +10,6 @@
 @_exported import Mixed
 @_exported import Mixed.Submodule
 
+@_exported import EmitWhileBuilding
+
 public func someFunc() {}