clang/test/Driver: Introduce the feature "clang-driver", set if gcc driver is not used.

It is not set at targetting cygming. See PR12920.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169824 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed