yaml2coff/elf: Touchup for compatibility.

* std::string::append(int, int) can be ambiguous.
* std::vector<>::data() is a C++11 feature, use ArrayRef abstraction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192542 91177308-0d34-0410-b5e6-96231b3b80d8