Wrap xar/xar.h include in extern "C" block

[glandium]

Summary

Without such wrapping, linking lld fails with missing symbols because of
C++ symbol mangling with older versions of the MacOSX SDK, in which
xar.h doesn't have an extern "C" block itself.

[thakis]

lg.

out of interest, which SDK version has this problem?

[glandium]

In D110224#3015751, @thakis wrote:

lg.

out of interest, which SDK version has this problem?

10.12 is apparently the last one with this problem. (yeah, I know, that's old)

lld/MachO/SyntheticSections.cpp

 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/EndianStream.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/Path.h"
 
 #ifdef LLVM_HAVE_LIBXAR
 #include <fcntl.h>
+extern "C" {
 #include <xar/xar.h>
+}
 #endif
 
 using namespace llvm;
 using namespace llvm::MachO;
 using namespace llvm::support;
 using namespace llvm::support::endian;
 using namespace lld;
 using namespace lld::macho;