[clang][ASTImporter] Add isNewDecl

Summary:
Add a new function with which we can query if a Decl had been newly
created during the import process. This feature is a must if we want to
have a different static analysis strategy for such newly created
declarations.

This is a dependent patch that is needed for the new CTU implementation
discribed at
https://discourse.llvm.org/t/rfc-much-faster-cross-translation-unit-ctu-analysis-implementation/61728

Differential Revision:
https://reviews.llvm.org/D123685

clang/include/clang/AST/ASTImporterSharedState.h

@@ -38,6 +38,9 @@ class ASTImporterSharedState {
   /// never cleared (like ImportedFromDecls).
   llvm::DenseMap<Decl *, ImportError> ImportErrors;
 
+  /// Set of the newly created declarations.
+  llvm::DenseSet<Decl *> NewDecls;
+
   // FIXME put ImportedFromDecls here!
   // And from that point we can better encapsulate the lookup table.
 
@@ -73,6 +76,10 @@ public:
   void setImportDeclError(Decl *To, ImportError Error) {
     ImportErrors[To] = Error;
   }
+
+  bool isNewDecl(const Decl *ToD) const { return NewDecls.count(ToD); }
+
+  void markAsNewDecl(Decl *ToD) { NewDecls.insert(ToD); }
 };
 
 } // namespace clang

clang/lib/AST/ASTImporter.cpp

@@ -285,6 +285,7 @@ namespace clang {
       ToD = CreateFun(std::forward<Args>(args)...);
       // Keep track of imported Decls.
       Importer.RegisterImportedDecl(FromD, ToD);
+      Importer.SharedState->markAsNewDecl(ToD);
       InitializeImportedDecl(FromD, ToD);
       return false; // A new Decl is created.
     }

clang/unittests/AST/ASTImporterTest.cpp

@@ -7698,6 +7698,38 @@ TEST_P(ASTImporterOptionSpecificTestBase,
   EXPECT_TRUE(ToX->getInClassInitializer());
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, isNewDecl) {
+  Decl *FromTU = getTuDecl(
+      R"(
+      int bar() {
+        return 0;
+      }
+      void other() {
+        bar();
+      }
+      )",
+      Lang_CXX11);
+  Decl *ToTU = getToTuDecl(
+      R"(
+      int bar() {
+        return 0;
+      }
+      )",
+      Lang_CXX11);
+  auto *FromOther = FirstDeclMatcher<FunctionDecl>().match(
+      FromTU, functionDecl(hasName("other")));
+  ASSERT_TRUE(FromOther);
+
+  auto *ToOther = Import(FromOther, Lang_CXX11);
+  ASSERT_TRUE(ToOther);
+
+  auto *ToBar = FirstDeclMatcher<FunctionDecl>().match(
+      ToTU, functionDecl(hasName("bar")));
+
+  EXPECT_TRUE(SharedStatePtr->isNewDecl(ToOther));
+  EXPECT_FALSE(SharedStatePtr->isNewDecl(ToBar));
+}
+
 INSTANTIATE_TEST_SUITE_P(ParameterizedTests, ASTImporterLookupTableTest,
                          DefaultTestValuesForRunOptions);