[llvm] [BPF] Support for `DW_TAG_variant_part` in BTF generation (PR #155783)

[Tamir Duberstein via llvm-commits]

================
@@ -672,16 +714,23 @@ void BTFDebug::visitStructType(const DICompositeType *CTy, bool IsStruct,
                                uint32_t &TypeId) {
   const DINodeArray Elements = CTy->getElements();
   uint32_t VLen = Elements.size();
+  // Variant parts have a discriminator. LLVM DI doesn't consider it as an
+  // element and instead keeps it as a separate reference. But we represent it
+  // as an element in BTF.
+  if (CTy->getTag() == dwarf::DW_TAG_variant_part)
+    VLen++;
----------------
tamird wrote:

Should this be nested to avoid a second `getDiscriminator()` call?

```c++
  
  if (CTy->getTag() == dwarf::DW_TAG_variant_part) {
    const auto *foo = CTy->getDiscriminator();
    if (foo) {
      visitTypeEntry(foo);
      VLen++;
    }
  }
```

https://github.com/llvm/llvm-project/pull/155783