[cxx-interop] Ensure that C++ structs are fully defined

Swift should never try to import C++ structs that are not fully defined. This is because Clang can't synthesize implicit members of such types, which might be necessary for Swift, e.g. a copy constructor.

This adds an assertion that makes sure that C++ structs are considered to be fully defined by Clang. This condition should already be satisfied.

Author: egorzhdan

lib/ClangImporter/ImportDecl.cpp

@@ -2764,8 +2764,8 @@ namespace {
       // with complete fields, and we're not in a dependent context(this is
       // equivalent to the logic in CanDeclareSpecialMemberFunction in Clang's
       // SemaLookup.cpp).
-      if (!decl->isBeingDefined() && !decl->isDependentContext() &&
-          areRecordFieldsComplete(decl)) {
+      assert(!decl->isBeingDefined() && "can only import fully defined decls");
+      if (!decl->isDependentContext() && areRecordFieldsComplete(decl)) {
         if (decl->hasInheritedConstructor()) {
           for (auto member : decl->decls()) {
             if (auto usingDecl = dyn_cast<clang::UsingDecl>(member)) {