[ASTImporter] Fix bug introduced in 2ba4e3a4

2ba4e3a4 (Move FPFeatures from BinaryOperator bitfields to Trailing
storage, D76384) introduced an assertion failure during CTU analysis.
The reason is that in ASTNodeImporter::VisitCompoundAssignOperator the
LHSType and the ResultType have been imported twice.

Details:
clang: ../../git/llvm-project/clang/lib/Basic/SourceManager.cpp:918: clang::FileID clang::SourceManager::getFileIDLoaded(unsigned int) const: Assertion `0 && "Invalid SLocOffset or bad function choice"' failed.
clang::SourceManager::getDecomposedExpansionLoc(clang::SourceLocation) const
clang::SourceManager::getPresumedLoc(clang::SourceLocation, bool) const
clang::ASTImporter::Import(clang::SourceLocation)
llvm::Error clang::ASTImporter::importInto<clang::SourceLocation>(clang::SourceLocation&, clang::SourceLocation const&)
clang::ASTNodeImporter::ImportDeclParts(clang::NamedDecl*, clang::DeclContext*&, clang::DeclContext*&, clang::DeclarationName&, clang::NamedDecl*&, clang::SourceLocation&)
clang::ASTNodeImporter::VisitRecordDecl(clang::RecordDecl*)
clang::declvisitor::Base<std::add_pointer, clang::ASTNodeImporter, llvm::Expected<clang::Decl*> >::Visit(clang::Decl*)
clang::ASTImporter::Import(clang::Decl*)
clang::ASTNodeImporter::VisitRecordType(clang::RecordType const*)
clang::TypeVisitor<clang::ASTNodeImporter, llvm::Expected<clang::QualType> >::Visit(clang::Type const*)
clang::ASTImporter::Import(clang::QualType)
clang::ASTNodeImporter::VisitElaboratedType(clang::ElaboratedType const*)
clang::TypeVisitor<clang::ASTNodeImporter, llvm::Expected<clang::QualType> >::Visit(clang::Type const*)
clang::ASTImporter::Import(clang::QualType)
clang::ASTNodeImporter::VisitPointerType(clang::PointerType const*)
clang::TypeVisitor<clang::ASTNodeImporter, llvm::Expected<clang::QualType> >::Visit(clang::Type const*)
clang::ASTImporter::Import(clang::QualType)
clang::QualType clang::ASTNodeImporter::importChecked<clang::QualType>(llvm::Error&, clang::QualType const&)
clang::ASTNodeImporter::VisitCompoundAssignOperator(clang::CompoundAssignOperator*)