This is the first step in adding sane error handling support to LLVMSystem.

The system API's will be shifted over to returning an error_code, and returning
other return values as out parameters to the function.

Code that needs to check error conditions will use the errc enum values which
are the same as the posix_errno defines (EBADF, E2BIG, etc...), and are
compatable with the error codes in WinError.h due to some magic in system_error.

An example would be:

if (error_code ec = KillEvil("Java")) { // error_code can be converted to bool.
  handle_error(ec);
}

llvm-svn: 119360