[SystemZ/ZOS] Add header file to encapsulate use of <sysexits.h>
The non-standard header file <sysexits.h> provides some return values.
EX_IOERR is used to as a special value to signal a broken pipe to the clang driver.
On z/OS Unix System Services, this header file does not exists. This patch
- adds a check for <sysexits.h>, removing the dependency on LLVM_ON_UNIX
- adds a new header file llvm/Support/ExitCodes, which either includes <sysexits.h> or defines EX_IOERR
- updates the users of EX_IOERR to include the new header file
Reviewed By: hubert.reinterpretcast
Differential Revision: https://reviews.llvm.org/D83472