size_t on Darwin AAPCS targets is "unsigned long". <rdar://problem/14136459>

Some embedded targets use ARM's AAPCS with iOS header files that define size_t
as unsigned long, which conflicts with the usual AAPCS definition of size_t
as unsigned int.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@184171 91177308-0d34-0410-b5e6-96231b3b80d8