[clang][NFC] remove unneeded nullptr checks after dereference (#100489)

Fix static verifer concerns of null pointer checks after dereferencing  
the pointer. Update the assert to make it super clear it is not null and
remove the checks.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -12248,16 +12248,15 @@ Decl *Sema::ActOnUsingEnumDeclaration(Scope *S, AccessSpecifier AS,
                                       SourceLocation EnumLoc, SourceRange TyLoc,
                                       const IdentifierInfo &II, ParsedType Ty,
                                       CXXScopeSpec *SS) {
-  assert(!SS->isInvalid() && "ScopeSpec is invalid");
+  assert(SS && !SS->isInvalid() && "ScopeSpec is invalid");
   TypeSourceInfo *TSI = nullptr;
   SourceLocation IdentLoc = TyLoc.getBegin();
   QualType EnumTy = GetTypeFromParser(Ty, &TSI);
   if (EnumTy.isNull()) {
-    Diag(IdentLoc, SS && isDependentScopeSpecifier(*SS)
+    Diag(IdentLoc, isDependentScopeSpecifier(*SS)
                        ? diag::err_using_enum_is_dependent
                        : diag::err_unknown_typename)
-        << II.getName()
-        << SourceRange(SS ? SS->getBeginLoc() : IdentLoc, TyLoc.getEnd());
+        << II.getName() << SourceRange(SS->getBeginLoc(), TyLoc.getEnd());
     return nullptr;
   }