...in "Menu: Tools > C++ > Tab: Code Model > Clang Code Model Warnings"
to avoid (re)parse issues due an invalid command line for libclang.

If an invalid option is detected, then indicate it by providing an error
message. As long as there is an invalid option, the options are not
accepted.

We do not want to maintain a white list of valid options, so mostly
check whether the option starts with "-W". An unknown or misspelled
option like "-WnotYetKnown" will be ignored by libclang.

To keep passing in some options for testing/debugging convenient, skip
validation if QTC_CLANG_NO_DIAGNOSTIC_CHECK is set.

Task-number: QTCREATORBUG-18864
Change-Id: I196d0474c7521969c54133f52dfc7be0394bfc3e
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>