[LinkerWrapper] Identify offloading sections using ELF type

Summary:
A previous patch added a new ELF section type for LLVM offloading. We
should use this when extracting the offloading sections rather than
checking the string. This pach also removes the implicit support for
COFF and MACH-O because we don't support those currently and should not
be included.

clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp

@@ -28,6 +28,7 @@
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/ArchiveWriter.h"
 #include "llvm/Object/Binary.h"
+#include "llvm/Object/ELFObjectFile.h"
 #include "llvm/Object/IRObjectFile.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Object/OffloadBinary.h"
@@ -339,9 +340,8 @@ Error extractOffloadFiles(MemoryBufferRef Contents,
 // Extract offloading binaries from an Object file \p Obj.
 Error extractFromBinary(const ObjectFile &Obj,
                         SmallVectorImpl<OffloadFile> &DeviceFiles) {
-  for (const SectionRef &Sec : Obj.sections()) {
-    Expected<StringRef> Name = Sec.getName();
-    if (!Name || !Name->equals(OFFLOAD_SECTION_MAGIC_STR))
+  for (ELFSectionRef Sec : Obj.sections()) {
+    if (Sec.getType() != ELF::SHT_LLVM_OFFLOADING)
       continue;
 
     Expected<StringRef> Buffer = Sec.getContents();
@@ -433,9 +433,7 @@ Error extractFromBuffer(std::unique_ptr<MemoryBuffer> Buffer,
   switch (Type) {
   case file_magic::bitcode:
     return extractFromBitcode(std::move(Buffer), DeviceFiles);
-  case file_magic::elf_relocatable:
-  case file_magic::macho_object:
-  case file_magic::coff_object: {
+  case file_magic::elf_relocatable: {
     Expected<std::unique_ptr<ObjectFile>> ObjFile =
         ObjectFile::createObjectFile(*Buffer, Type);
     if (!ObjFile)