Uses generic_category instead of system_category.

Some c++ libraries (libstdc++ at least) don't seem to map to the generic
category in in the system_category's default_error_condition.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210635 91177308-0d34-0410-b5e6-96231b3b80d8