Fix a comparison of integers of different signs warning.

  source/Plugins/DynamicLoader/Darwin-Kernel/DynamicLoaderDarwinKernel.cpp:403:21: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  for (int i = 0; i < llvm::array_lengthof (magicks); i++)
                  ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Differential Revision: https://reviews.llvm.org/D27081

llvm-svn: 287848