[CMake] [PR27403] Fix COMPILER_RT_ENABLE_IOS when using Xcode from the App Store.

This change modifies find_darwin_sdk_dir to set a variable if a Darwin "Internal" SDK is present which allows CMake to disable components that require internal-only APIs.

This mechanism is then used to disable TSan when an internal SDK is not present.

llvm-svn: 267575
2 files changed