[mach-o] Add support for reading utf16 string literal sections

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209684 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] Add checks that string literals in object files are zero terminated

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209685 91177308-0d34-0410-b5e6-96231b3b80d8