Use std::error_code instead of llvm::error_code.
The idea of this patch is to turn llvm/Support/system_error.h into a
transitional header that just brings in the erorr_code api to the llvm
namespace. I will remove it shortly afterwards.
The cases where the general idea needed some tweaking:
- std::errc is a namespace in msvc, so we cannot use "using std::errc". I could
add an #ifdef, but there were not that many uses, so I just added std:: to
them in this patch.
- Template specialization had to be moved to the std namespace in this
patch set already.
- The msvc implementation of default_error_condition doesn't seem to
provide the same transformations as we need. Not too surprising since
the standard doesn't actually say what "equivalent" means. I fixed the
problem by keeping our old mapping and using it at error_code
Despite these shortcomings I think this is still a good thing. Some reasons:
- The different implementations of system_error might improve over time.
- It removes 925 lines of code from llvm already.
- It removes 6313 bytes from the text segment of the clang binary when
it is built with gcc and 2816 bytes when building with clang and