Openthread build issues with clang/llvm

[nashif]

When building openthread tests with llvm/clang, I get the following:

/home/nashif/Work/zephyrproject/zephyr/tests/subsys/openthread/radio_test.c:426:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        otPlatRadioSetTransmitPower(ot, power);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~
/home/nashif/Work/zephyrproject/zephyr/tests/subsys/openthread/radio_test.c:432:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        otPlatRadioReceive(ot, chan);
        ^~~~~~~~~~~~~~~~~~ ~~~~~~~~
/home/nashif/Work/zephyrproject/zephyr/tests/subsys/openthread/radio_test.c:493:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        otPlatRadioSetTransmitPower(ot, -3);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~
/home/nashif/Work/zephyrproject/zephyr/tests/subsys/openthread/radio_test.c:494:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        otPlatRadioGetTransmitPower(ot, &out_power);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~
/home/nashif/Work/zephyrproject/zephyr/tests/subsys/openthread/radio_test.c:496:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        otPlatRadioSetTransmitPower(ot, -6);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~
/home/nashif/Work/zephyrproject/zephyr/tests/subsys/openthread/radio_test.c:497:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        otPlatRadioGetTransmitPower(ot, &out_power);

We should be using the result of otPlatRadioGetTransmitPower

We need a clean clang/llvm build to be able to integrate this in CI.

This was reproduced with:

export ZEPHYR_TOOLCHAIN_VARIANT=llvm
sanitycheck -p native_posix