[mlir][Linalg] Fix ignoring nodiscard return value

[tblah]

Summary

ff94419a287c changed the return value of appendMangledType() to
LogicalResult, which is marked as nodiscard. Ignoring the result
generates a warning when building with clang.

[nicolasvasilache]

Thanks for the fix, I landed it in @b0d575310f2ee660dadfdaf1e06bac148a575754 to trigger a round of postsubmits.

mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp

 static LogicalResult appendMangledType(llvm::raw_string_ostream &ss, Type t) {
   if (auto memref = t.dyn_cast<MemRefType>()) {
     ss << "view";
     for (auto size : memref.getShape())
       if (size < 0)
         ss << "sx";
       else
         ss << size << "x";
-    appendMangledType(ss, memref.getElementType());
+    if (failed(appendMangledType(ss, memref.getElementType())))
+      return failure();
     return success();
   }
   if (auto vec = t.dyn_cast<VectorType>()) {
     ss << "vector";
     llvm::interleave(
         vec.getShape(), [&](int64_t i) { ss << i; }, [&]() { ss << "x"; });
-    appendMangledType(ss, vec.getElementType());
+    if (failed(appendMangledType(ss, vec.getElementType())))
+      return failure();
     return success();
   } else if (t.isSignlessIntOrIndexOrFloat()) {
     ss << t;
     return success();
   }
   return failure();
 }