Fix quadratic runtime for duplicate export name detection #2120

Job	Run time
build_llvm_libraries_xtensa / build_llvm_libraries	2m 56s
build_llvm_libraries / build_llvm_libraries	2m 56s
spec_test_on_qemu (boards/arm/imx6/sabre-6quad/configs/nsh, thumbv7_vfp, dp, -t aot, CONFIG_INTER...	10m 52s
spec_test_on_qemu (boards/arm/imx6/sabre-6quad/configs/nsh, thumbv7_vfp, dp, -t aot, CONFIG_INTER...	11m 58s
spec_test_on_qemu (boards/arm/imx6/sabre-6quad/configs/nsh, thumbv7_vfp, dp, -t aot -X, CONFIG_IN...	10m 57s
spec_test_on_qemu (boards/arm/imx6/sabre-6quad/configs/nsh, thumbv7_vfp, dp, -t aot -X, CONFIG_IN...	12m 4s
spec_test_on_qemu (boards/risc-v/qemu-rv/rv-virt/configs/nsh, riscv32, none, -t aot, CONFIG_INTER...	8m 39s
spec_test_on_qemu (boards/risc-v/qemu-rv/rv-virt/configs/nsh, riscv32, none, -t aot, CONFIG_INTER...	9m 15s
spec_test_on_qemu (boards/risc-v/qemu-rv/rv-virt/configs/nsh, riscv32, none, -t aot -X, CONFIG_IN...	8m 33s
spec_test_on_qemu (boards/risc-v/qemu-rv/rv-virt/configs/nsh, riscv32, none, -t aot -X, CONFIG_IN...	9m 13s
spec_test_on_qemu (boards/risc-v/qemu-rv/rv-virt/configs/nsh64, riscv64, none, -t aot, CONFIG_INT...	8m 46s
spec_test_on_qemu (boards/risc-v/qemu-rv/rv-virt/configs/nsh64, riscv64, none, -t aot, CONFIG_INT...	9m 15s
spec_test_on_qemu (boards/xtensa/esp32s3/esp32s3-devkit/configs/qemu_debug, xtensa, none, -t aot,...	12m 33s
spec_test_on_qemu (boards/xtensa/esp32s3/esp32s3-devkit/configs/qemu_debug, xtensa, none, -t aot ...	12m 21s
	2h 10m 18s

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