PDB/BinaryStreamTest.cpp: Appease mingw to avoid std::errc::no_buffer_space.

Unfortunately, mingw's libstdc++ doesn't provide winsock2 errors.
That said, we should avoid raising OS-oriented error code in our code.

For now, I suggest to define custom error from std::error_category.
See also; https://reviews.llvm.org/D20592

llvm-svn: 296581