[llvm] [Object,ELF] Implement PN_XNUM extension for program headers (PR #162288)

[via llvm-commits]

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@@ -889,7 +887,31 @@ Expected<uint64_t> ELFFile<ELFT>::getDynSymtabSize() const {
   return 0;
 }
 
-template <class ELFT> ELFFile<ELFT>::ELFFile(StringRef Object) : Buf(Object) {}
+template <class ELFT> ELFFile<ELFT>::ELFFile(StringRef Object) : Buf(Object) {
+  const Elf_Ehdr &Header = getHeader();
+  RealPhNum = Header.e_phnum;
+  RealShNum = Header.e_shnum;
+  RealShStrNdx = Header.e_shstrndx;
+  if (!Header.hasPhdrNumExtension())
+    return;
+
+  // An ELF binary may report `hasExtendedHeader` as true but not actually
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aokblast wrote:

Should we use std::optional for the new getter?
Or Real_* ? *Real_* : getHeader()->e_*.

https://github.com/llvm/llvm-project/pull/162288