Interop support for nullptr and nullptr_t. (#6353)

Add a `Core.CppCompat.NullptrT` type that C++'s `nullptr_t` maps into.
Map `nullptr` to an uninitialized constant of that type -- `nullptr`
doesn't actually have any defined bits within it, despite having the
same representation as `void*`.

toolchain/sem_ir/expr_info.cpp

@@ -166,6 +166,7 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
       case TypeType::Kind:
       case UnaryOperatorNot::Kind:
       case UnboundElementType::Kind:
+      case UninitializedValue::Kind:
       case ValueOfInitializer::Kind:
       case ValueParam::Kind:
       case VtableType::Kind: