clang-cl: ignore ths /GS- flag

The /GS- flag is used to turn off run-time buffer security checks (/GS).
Since no such checks are enabled in the first place, I think we should just
ignore this flag.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190900 91177308-0d34-0410-b5e6-96231b3b80d8