[CMake] Teach the build system to codesign built products

Automatically codesign all executables and dynamic libraries if a
codesigning identity is given (via LLVM_CODESIGNING_IDENTITY). This
option is darwin only for now.

Also update platforms/iOS.cmake to pick up the right versions of
codesign and codesign_allocate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336708 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed