[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
diff --git a/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake b/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake
index 70a67ca..0c1637d 100644
--- a/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake
+++ b/compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake
@@ -16,6 +16,8 @@
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_FILE /dev/null
     )
+  else()
+    set(${var}_INTERNAL ${var_internal} PARENT_SCOPE)
   endif()
   set(${var} ${var_internal} PARENT_SCOPE)
 endfunction()