[clang] [llvm] [Offloading] Extend OffloadBinary format to support multiple metadata entries (PR #169425)

[Yury Plyakhin via llvm-commits]

================
@@ -181,7 +195,7 @@ class OffloadFile : public OwningBinary<OffloadBinary> {
         getBinary()->getMemoryBufferRef().getBufferIdentifier());
 
     // This parsing should never fail because it has already been parsed.
-    auto NewBinaryOrErr = OffloadBinary::create(*Buffer);
+    auto NewBinaryOrErr = OffloadBinary::createV1(*Buffer);
----------------
YuriPlyakhin wrote:

OffloadFile::copy is a deep copy used to create a copy of OffloadBinary for specific target (to split binaries into sets of compatible target binaries). Currently copy method is using OffloadBinary::create to create such Binary from a Buffer.

In new OffloadBinary format Buffer may contain multiple entries/images for non-compatible targets. Per my understanding of usage model, we still want to be able to do an OffloadFile:copy for just 1 single entry/image (even if buffer would be shared between multiple incompatible target binaries) and hence create method should be able to create an OffloadFile, which would have shared buffer with other OffloadFile objects, but it would contain only 1 entry/image pointer or something.

Hence, I'm thinking of creating something like this: 
```
  LLVM_ABI static Expected<std::unique_ptr<OffloadBinary>>
      create(MemoryBufferRef Buf, index Num = 0);
```
So, for old OffloadBinary, we would not need index, but for new OffloadBinary we would be able to create one OffloadBinary with shared buffer.
Hence extractOffloadFiles creates multiple binaries (either with own buffer each for old format or with shared buffer between binaries for new format)

https://github.com/llvm/llvm-project/pull/169425