[llvm-nm] Fix heap-use-after-free while executing 'llvm-nm -n --export-symbols' (#65668)

Use symbol's flags saved in `NMSymbol::SymFlags` inside
`NMSymbol::isDefined()` since `BasicSymbolRef::getFlags()` requires the
symbol's containing entity object to exist (which doesn't, causing
llvm-nm to crash).

Author: chbessonova

llvm/tools/llvm-nm/llvm-nm.cpp

@@ -237,10 +237,8 @@ struct NMSymbol {
   std::string IndirectName;
 
   bool isDefined() const {
-    if (Sym.getRawDataRefImpl().p) {
-      uint32_t Flags = cantFail(Sym.getFlags());
-      return !(Flags & SymbolRef::SF_Undefined);
-    }
+    if (Sym.getRawDataRefImpl().p)
+      return !(SymFlags & SymbolRef::SF_Undefined);
     return TypeChar != 'U';
   }