[MLIR] Specify namespace for standard dialect using cppNamespace field

The `::mlir` namespace for operations from standard is currently
defined by enclosing the header file generated from the ODS in
`Ops.td` in a namespace in `Ops.h`. However, when referencing
operations from `Ops.td` in other TableGen files, this causes the
generated C++ code to refer to classes from the global namespace
instead of `::mlir`.

By defining the namespace through the `cppNamespace` field for
`StandardOps_Dialect` directly in `Ops.td` instead, the ODS
becomes reusable in other TableGen files through simple
inclusion.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D103234

mlir/include/mlir/Dialect/StandardOps/IR/Ops.h

@@ -41,12 +41,14 @@ class PatternRewriter;
 /// with `b` at location `loc`.
 SmallVector<Range, 8> getOrCreateRanges(OffsetSizeAndStrideOpInterface op,
                                         OpBuilder &b, Location loc);
+} // namespace mlir
 
 #define GET_OP_CLASSES
 #include "mlir/Dialect/StandardOps/IR/Ops.h.inc"
 
 #include "mlir/Dialect/StandardOps/IR/OpsDialect.h.inc"
 
+namespace mlir {
 /// This is a refinement of the "constant" op for the case where it is
 /// returning a float value of FloatType.
 ///

mlir/include/mlir/Dialect/StandardOps/IR/Ops.td

@@ -25,7 +25,7 @@ include "mlir/Interfaces/ViewLikeInterface.td"
 
 def StandardOps_Dialect : Dialect {
   let name = "std";
-  let cppNamespace = "";
+  let cppNamespace = "::mlir";
   let hasConstantMaterializer = 1;
   // TODO: This dependency is needed to handle memref ops in the
   // canonicalize pass and should be resolved.