[MLIR] Fix missing return statement warning in PatternMatch.h

Fix missing return statement warning in PatternMatch.h. NFC.

```
mlir/include/mlir/IR/PatternMatch.h:983:3: warning: no return statement in
function returning non-void [-Wreturn-type]
```

Differential Revision: https://reviews.llvm.org/D123756

diff --git a/mlir/include/mlir/IR/PatternMatch.h b/mlir/include/mlir/IR/PatternMatch.h
index aa67709..751ec2a 100644 (file)
--- a/mlir/include/mlir/IR/PatternMatch.h
+++ b/mlir/include/mlir/IR/PatternMatch.h
@@ -980,6 +980,7 @@ struct ProcessPDLValue<std::string>
     static_assert(always_false<T>,
                   "`std::string` arguments require a string copy, use "
                   "`StringRef` for string-like arguments instead");
+    return {};
   }
   static void processAsResult(PatternRewriter &rewriter, PDLResultList &results,
                               StringRef value) {