Silence gcc warning "control reaches end of non-void function" [NFCI]

Without this fix gcc (7.4) complains with

 /data/repo/master/clang/lib/CodeGen/CGObjCMac.cpp: In member function 'std::__cxx11::string {anonymous}::CGObjCCommonMac::GetSectionName(llvm::StringRef, llvm::StringRef)':
 /data/repo/master/clang/lib/CodeGen/CGObjCMac.cpp:4944:1: error: control reaches end of non-void function [-Werror=return-type]
  }
  ^

All values in the ObjectFormatType enum are currently handled in the switch
but gcc complains anyway.

llvm-svn: 365174