[MLIR] Fix missing ::mlir namespace

[ataheridezfouli-groq]

Summary

Add a missing ::mlir namespace for MixedContainerType
that only triggers if the user is not using namespace mlir.

[ataheridezfouli-groq]

@rriddle do you mind merging this for me. I don't have commit privileges. Thank you.

mlir/include/mlir/IR/OpBase.td

 // ContainerType) can hold elements with a mix of types. Requires a call that
 // produces a list of all elements' types.
 class MixedContainerType<Type etype, Pred containerPred, code elementTypesCall,
                          string descr> :
     Type<
         And<[
             containerPred,
             Concat<
-                "::llvm::all_of(" # elementTypesCall # ", [](Type t) { "
+                "::llvm::all_of(" # elementTypesCall # ", [](::mlir::Type t) { "
                 "return t && (",
                 SubstLeaves<"$_self", "t", etype.predicate>,
                 "); })"
             >
         ]>,
         descr # " with any combination of " # etype.summary # " values"> {
   // The type of elements in the container.
   Type elementType = etype;