[ClangImporter] Assert when we're expecting a definition (#11170)

No functionality change.

lib/ClangImporter/ImportEnumInfo.cpp

@@ -36,6 +36,8 @@ using namespace importer;
 /// Classify the given Clang enumeration to describe how to import it.
 void EnumInfo::classifyEnum(ASTContext &ctx, const clang::EnumDecl *decl,
                             clang::Preprocessor &pp) {
+  assert(decl->isThisDeclarationADefinition());
+
   // Anonymous enumerations simply get mapped to constants of the
   // underlying type of the enum, because there is no way to conjure up a
   // name for the Swift type.

lib/ClangImporter/ImportName.cpp

@@ -587,6 +587,13 @@ static bool matchesVersion(A *versionedAttr, ImportNameVersion version) {
 const clang::SwiftNameAttr *
 importer::findSwiftNameAttr(const clang::Decl *decl,
                             ImportNameVersion version) {
+#ifndef NDEBUG
+  if (Optional<const clang::Decl *> def = getDefinitionForClangTypeDecl(decl)) {
+    assert((*def == nullptr || *def == decl) &&
+           "swift_name should only appear on the definition");
+  }
+#endif
+
   if (version == ImportNameVersion::Raw)
     return nullptr;