[mlir:OpFormatGen] Add Support for $_ctxt in the transformer.

This is useful for "build tuple" type ops. In my case, in npcomp, I have
an op:

```
// Result type is `!torch.tuple<!torch.tensor, !torch.tensor>`.
torch.prim.TupleConstruct %0, %1 : !torch.tensor, !torch.tensor
```

and the context is required for the `Torch::TupleType::get` call (for
the case of an empty tuple).

The handling of these FmtContext's in the code is pretty ad-hoc -- I didn't
attempt to rationalize it and just made a targeted fix. As someone
unfamiliar with the code I had a hard time seeing how to more broadly fix
the situation.

Differential Revision: https://reviews.llvm.org/D104274

mlir/test/lib/Dialect/Test/TestOps.td

@@ -1876,6 +1876,15 @@ def FormatTypesMatchAttrOp : TEST_Op<"format_types_match_attr", [
   let assemblyFormat = "attr-dict $value";
 }
 
+def FormatTypesMatchContextOp : TEST_Op<"format_types_match_context", [
+    TypesMatchWith<"tuple result type matches operand type", "value", "result",
+        "::mlir::TupleType::get($_ctxt, $_self)">
+  ]> {
+  let arguments = (ins AnyType:$value);
+  let results = (outs AnyType:$result);
+  let assemblyFormat = "attr-dict $value `:` type($value)";
+}
+
 //===----------------------------------------------------------------------===//
 // Test SideEffects
 //===----------------------------------------------------------------------===//

mlir/test/mlir-tblgen/op-format.mlir

@@ -348,3 +348,6 @@ test.format_infer_variadic_type_from_non_variadic %i64, %i64 : i64
 
 // CHECK: test.format_types_match_attr 1 : i64
 %ignored_res5 = test.format_types_match_attr 1 : i64
+
+// CHECK: test.format_types_match_context %[[I64]] : i64
+%ignored_res6 = test.format_types_match_context %i64 : i64

mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp

@@ -579,6 +579,7 @@ OpEmitter::OpEmitter(const Operator &op,
       opClass(op.getCppClassName(), op.getExtraClassDeclaration()),
       staticVerifierEmitter(staticVerifierEmitter) {
   verifyCtx.withOp("(*this->getOperation())");
+  verifyCtx.addSubst("_ctxt", "this->getOperation()->getContext()");
 
   genTraits();
 

mlir/tools/mlir-tblgen/OpFormatGen.cpp

@@ -1343,6 +1343,7 @@ void OperationFormat::genParserTypeResolution(Operator &op,
     } else if (const NamedTypeConstraint *var = resolver.getVariable()) {
       if (Optional<StringRef> tform = resolver.getVarTransformer()) {
         FmtContext fmtContext;
+        fmtContext.addSubst("_ctxt", "parser.getBuilder().getContext()");
         if (var->isVariadic())
           fmtContext.withSelf(var->name + "Types");
         else