Add API for adding arguments to blocks

[GeorgeLyon]

Summary

This just exposes a missing API

[stellaraccident]

Thanks. I thought we had this but was obviously misremembering.

[stellaraccident]

Can you please extend the test to exercise this?

[GeorgeLyon]

It looks like there are three tests failing currently on master for me, but they also fail for the previous commit (8d73bee4edc2)
[build] Failed Tests (3):
[build] MLIR :: mlir-cpu-runner/async-group.mlir
[build] MLIR :: mlir-cpu-runner/async-value.mlir
[build] MLIR :: mlir-cpu-runner/async.mlir

[GeorgeLyon]

Add test

mlir/include/mlir-c/IR.h

 /// the operation. Otherwise, the reference must belong to the block.
 MLIR_CAPI_EXPORTED void
 mlirBlockInsertOwnedOperationBefore(MlirBlock block, MlirOperation reference,
                                     MlirOperation operation);
 
 /// Returns the number of arguments of the block.
 MLIR_CAPI_EXPORTED intptr_t mlirBlockGetNumArguments(MlirBlock block);
 
+/// Appends an argument of the specified type to the block. Returns the newly
+/// added argument.
+MLIR_CAPI_EXPORTED MlirValue mlirBlockAddArgument(MlirBlock block,
+                                                  MlirType type);
+
 /// Returns `pos`-th argument of the block.
 MLIR_CAPI_EXPORTED MlirValue mlirBlockGetArgument(MlirBlock block,
                                                   intptr_t pos);
 
 /// Prints a block by sending chunks of the string representation and
 /// forwarding `userData to `callback`. Note that the callback may be called
 /// several times with consecutive chunks of the string.
 MLIR_CAPI_EXPORTED void

mlir/lib/CAPI/IR/IR.cpp

 }
 
 void mlirBlockDestroy(MlirBlock block) { delete unwrap(block); }
 
 intptr_t mlirBlockGetNumArguments(MlirBlock block) {
   return static_cast<intptr_t>(unwrap(block)->getNumArguments());
 }
 
+MlirValue mlirBlockAddArgument(MlirBlock block, MlirType type) {
+  return wrap(unwrap(block)->addArgument(unwrap(type)));
+}
+
 MlirValue mlirBlockGetArgument(MlirBlock block, intptr_t pos) {
   return wrap(unwrap(block)->getArgument(static_cast<unsigned>(pos)));
 }
 
 void mlirBlockPrint(MlirBlock block, MlirStringCallback callback,
                     void *userData) {
   detail::CallbackOstream stream(callback, userData);
   unwrap(block)->print(stream);

mlir/test/CAPI/ir.c

MlirModule makeAndDumpAdd(MlirContext ctx, MlirLocation location) {
   MlirOperationState dimState = mlirOperationStateGet(
       mlirStringRefCreateFromCString("std.dim"), location);
   mlirOperationStateAddOperands(&dimState, 2, dimOperands);
   mlirOperationStateAddResults(&dimState, 1, &indexType);
   MlirOperation dim = mlirOperationCreate(&dimState);
   mlirBlockAppendOwnedOperation(funcBody, dim);
 
   MlirRegion loopBodyRegion = mlirRegionCreate();
-  MlirBlock loopBody = mlirBlockCreate(/*nArgs=*/1, &indexType);
+  MlirBlock loopBody = mlirBlockCreate(0, NULL);
+  mlirBlockAddArgument(loopBody, indexType);
   mlirRegionAppendOwnedBlock(loopBodyRegion, loopBody);
 
   MlirAttribute indexOneLiteral =
       mlirAttributeParseGet(ctx, mlirStringRefCreateFromCString("1 : index"));
   MlirNamedAttribute indexOneValueAttr = mlirNamedAttributeGet(
       mlirIdentifierGet(ctx, mlirStringRefCreateFromCString("value")),
       indexOneLiteral);
   MlirOperationState constOneState = mlirOperationStateGet(