Revert "Allow "[[FLAGS=<none>]]" value in the ELF Fileheader Flags field (#143845)"

This reverts commit 0b054e2.
Breaks many tests, see comments on #143845.

Author: nico

llvm/include/llvm/ObjectYAML/ELFYAML.h

@@ -117,7 +117,7 @@ struct FileHeader {
   llvm::yaml::Hex8 ABIVersion;
   ELF_ET Type;
   std::optional<ELF_EM> Machine;
-  std::optional<ELF_EF> Flags;
+  ELF_EF Flags;
   llvm::yaml::Hex64 Entry;
   std::optional<StringRef> SectionHeaderStringTable;
 

llvm/lib/ObjectYAML/ELFEmitter.cpp

@@ -481,11 +481,7 @@ void ELFState<ELFT>::writeELFHeader(raw_ostream &OS) {
 
   Header.e_version = EV_CURRENT;
   Header.e_entry = Doc.Header.Entry;
-  if (Doc.Header.Flags)
-    Header.e_flags = *Doc.Header.Flags;
-  else
-    Header.e_flags = 0;
-
+  Header.e_flags = Doc.Header.Flags;
   Header.e_ehsize = sizeof(Elf_Ehdr);
 
   if (Doc.Header.EPhOff)

llvm/test/tools/obj2yaml/ELF/eflags.yaml

@@ -281,8 +281,7 @@ template <class ELFT> Expected<ELFYAML::Object *> ELFDumper<ELFT>::dump() {
   Y->Header.Type = Obj.getHeader().e_type;
   if (Obj.getHeader().e_machine != 0)
     Y->Header.Machine = ELFYAML::ELF_EM(Obj.getHeader().e_machine);
-  if (Obj.getHeader().e_flags != 0)
-    Y->Header.Flags = Obj.getHeader().e_flags;
+  Y->Header.Flags = Obj.getHeader().e_flags;
   Y->Header.Entry = Obj.getHeader().e_entry;
 
   // Dump sections