Explicitly specify the default toolchain in test suite on Darwin.
Explicitly specify the default toolchain in test suite on Darwin.

If CC is not specified, default to specifying the default
toolchain manually.

Workaround for:
rdar://problem/23556268

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb