[mlir] Simplify unreachable type switch cases. NFC. (#162032)

Use `DefaultUnreachable` from
llvm/llvm-project#161970.

Author: kuhar

mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp

@@ -89,10 +89,7 @@ static ParameterElement *getEncapsulatedParameterElement(FormatElement *el) {
       .Case<ParameterElement>([&](auto param) { return param; })
       .Case<RefDirective>(
           [&](auto ref) { return cast<ParameterElement>(ref->getArg()); })
-      .Default([&](auto el) {
-        assert(false && "unexpected struct element type");
-        return nullptr;
-      });
+      .DefaultUnreachable("unexpected struct element type");
 }
 
 /// Shorthand functions that can be used with ranged-based conditions.