Cannot build Clang etc. on Windows in Debug Mode

(Originally sent to the cfe-users List, but someone said it is probably
better to post it here)

Your ninja output should have a line containing the word "FAILED" which tells you the exact command that failed. The build is highly parallel, so that line can get buried; searching for "FAILED" is the best way to find it.

Also, ninja is parallel by default, so you don't need to specify -j%NUMBER_OF_PROCESSORS% explicitly.