Pass the computed magic to createBinary and createObjectFile if available.

identify_magic is not free, so we should avoid calling it twice. The argument
also makes it cheap for createBinary to just forward to createObjectFile.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199813 91177308-0d34-0410-b5e6-96231b3b80d8