clang-cl: ignore /RTC options (PR17129)

I don't think Clang intends to implement this functionality.
ASan should be used instead. Since /RTC is often passed by default
from MSBuild, ignore the option to avoid bloating the output.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190202 91177308-0d34-0410-b5e6-96231b3b80d8