Revert "[GH54588]Fix ItaniumMangler for NTTP unnamed unions w/ unname…

…d structs"

This reverts commit 4cf98f9.
The test fails on mac bots, see comments on https://reviews.llvm.org/D122820

Also reverts follow-ups eb92098 and
861c189.

clang/lib/AST/ItaniumMangle.cpp

@@ -5545,47 +5545,6 @@ static QualType getLValueType(ASTContext &Ctx, const APValue &LV) {
   return T;
 }
 
-static IdentifierInfo *getUnionInitName(SourceLocation UnionLoc,
-                                        DiagnosticsEngine &Diags,
-                                        const FieldDecl *FD) {
-  // According to:
-  // http://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling.anonymous
-  // For the purposes of mangling, the name of an anonymous union is considered
-  // to be the name of the first named data member found by a pre-order,
-  // depth-first, declaration-order walk of the data members of the anonymous
-  // union.
-
-  if (FD->getIdentifier())
-    return FD->getIdentifier();
-
-  // The only cases where the identifer of a FieldDecl would be blank is if the
-  // field represents an anonymous record type or if it is an unnamed bitfield.
-  // There is no type to descend into in the case of a bitfield, so we can just
-  // return nullptr in that case.
-  if (FD->isBitField())
-    return nullptr;
-  const CXXRecordDecl *RD = FD->getType()->getAsCXXRecordDecl();
-
-  // Consider only the fields in declaration order, searched depth-first.  We
-  // don't care about the active member of the union, as all we are doing is
-  // looking for a valid name. We also don't check bases, due to guidance from
-  // the Itanium ABI folks.
-  for (const FieldDecl *RDField : RD->fields()) {
-    if (IdentifierInfo *II = getUnionInitName(UnionLoc, Diags, RDField))
-      return II;
-  }
-
-  // According to the Itanium ABI: If there is no such data member (i.e., if all
-  // of the data members in the union are unnamed), then there is no way for a
-  // program to refer to the anonymous union, and there is therefore no need to
-  // mangle its name. However, we should diagnose this anyway.
-  unsigned DiagID = Diags.getCustomDiagID(
-      DiagnosticsEngine::Error, "cannot mangle this unnamed union NTTP yet");
-  Diags.Report(UnionLoc, DiagID);
-
-  return nullptr;
-}
-
 void CXXNameMangler::mangleValueInTemplateArg(QualType T, const APValue &V,
                                               bool TopLevel,
                                               bool NeedExactType) {
@@ -5669,10 +5628,7 @@ void CXXNameMangler::mangleValueInTemplateArg(QualType T, const APValue &V,
     mangleType(T);
     if (!isZeroInitialized(T, V)) {
       Out << "di";
-      IdentifierInfo *II = (getUnionInitName(
-          T->getAsCXXRecordDecl()->getLocation(), Context.getDiags(), FD));
-      if (II)
-        mangleSourceName(II);
+      mangleSourceName(FD->getIdentifier());
       mangleValueInTemplateArg(FD->getType(), V.getUnionValue(), false);
     }
     Out << 'E';