[mlir] [llvm] [mlir] Add config for PDL (PR #69927)

[Mehdi Amini via llvm-commits]

================
@@ -1863,6 +944,8 @@ class RewritePatternSet {
 
   MLIRContext *const context;
   NativePatternListT nativePatterns;
+
+  // Technically not needed if PDL not enabled. Stubs enable less ifdefs.
----------------
joker-eph wrote:

```suggestion
  /// Patterns expressed with PDL. This will compile to a stub class when PDL is not enabled.
```

https://github.com/llvm/llvm-project/pull/69927