[CMake] Use just-built clang and build iOS support when building stage2

The Apple stage2 build should include compiler-rt iOS libraries and be built with the stage2 compiler. This matches Apple's production clang builds.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267584 91177308-0d34-0410-b5e6-96231b3b80d8