Remove system_error.h.

This is a minimal change to remove the header. I will remove the occurrences
of "using std::error_code" in a followup patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210803 91177308-0d34-0410-b5e6-96231b3b80d8
Trying to fix the windows build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210805 91177308-0d34-0410-b5e6-96231b3b80d8