[Support] Add missing includes of <system_error> for std::error_code

This fixes compilation with MinGW after SVN r333798, which added
a few functions within _WIN32 ifdefs, functions returning
std::error_code. Include everything that is needed instead of
hoping that this header being inclued transitively (which it apparently
is in MSVC builds).

Change-Id: I9a73bbb9186910d1064138f31f741085290b9468
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335107 91177308-0d34-0410-b5e6-96231b3b80d8