[cxx-interop] Pass sysroot to the Clang driver when detecting libstdc++ path
[cxx-interop] Pass sysroot to the Clang driver when detecting libstdc++ path

The Swift build scripts pass `-sdk /` to the Swift compiler when building the Swift stdlib and the overlays, and this affects the default include paths of Clang. We need to make sure that libstdc++ path detected by ClangImporter is the same path that will be used by Clang during compilation. To do that, let's pass `--sysroot` to the Clang driver when detecting the libstdc++ path.

roambotics/swift,apple/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,apple/swift,roambotics/swift,JGiola/swift,roambotics/swift,JGiola/swift,roambotics/swift,JGiola/swift,roambotics/swift,JGiola/swift,glessard/swift,glessard/swift,JGiola/swift,apple/swift,glessard/swift,apple/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,apple/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,apple/swift,JGiola/swift,roambotics/swift