Revert "raw_ostream: add operator<< overload for std::error_code"

This reverts commit r368849, because it breaks some bots (e.g.
llvm-clang-x86_64-win-fast).

It turns out this is not as NFC as we had hoped, because operator== will
consider two std::error_codes to be distinct even though they both hold
"success" values if they have different categories.

llvm-svn: 368854