[flang][NFC] move fir namespace into FortranVariableOpInterface ODS declaration

[jeanPerier]

Summary

This will be needed to be able to use this interface in HLFIR ODS operation definition
and get the right namespaces.

[clementval]

LGTM

flang/include/flang/Optimizer/Dialect/FIROps.td

def fir_IsPresentOp : fir_SimpleOp<"is_present", [NoMemoryEffect]> {
   }];
 
   let arguments = (ins AnyRefOrBoxLike:$val);
 
   let results = (outs BoolLike);
 }
 
 def fir_DeclareOp : fir_Op<"declare", [AttrSizedOperandSegments,
-    DeclareOpInterfaceMethods<FortranVariableOpInterface>]> {
+    DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable";
 
   let description = [{
     Tie the properties of a Fortran variable to an address. The properties
     include bounds, length parameters, and Fortran attributes.
 
     The memref argument describes the storage of the variable. It may be a
     raw address (fir.ref<T>), or a box or class value or address (fir.box<T>,

flang/include/flang/Optimizer/Dialect/FortranVariableInterface.h

 #ifndef FORTRAN_OPTIMIZER_DIALECT_FORTRANVARIABLEINTERFACE_H
 #define FORTRAN_OPTIMIZER_DIALECT_FORTRANVARIABLEINTERFACE_H
 
 #include "flang/Optimizer/Dialect/FIRAttr.h"
 #include "flang/Optimizer/Dialect/FIRType.h"
 #include "mlir/IR/BuiltinTypes.h"
 #include "mlir/IR/OpDefinition.h"
 
-namespace fir {
 #include "flang/Optimizer/Dialect/FortranVariableInterface.h.inc"
-} // namespace fir
 
 #endif // FORTRAN_OPTIMIZER_DIALECT_FORTRANVARIABLEINTERFACE_H

flang/include/flang/Optimizer/Dialect/FortranVariableInterface.td

 //===----------------------------------------------------------------------===//
 
 #ifndef FORTRANVARIABLEINTERFACE
 #define FORTRANVARIABLEINTERFACE
 
 include "mlir/IR/OpBase.td"
 
 
-def FortranVariableOpInterface : OpInterface<"FortranVariableOpInterface"> {
+def fir_FortranVariableOpInterface : OpInterface<"FortranVariableOpInterface"> {
   let description = [{
     Interface for operations that create Fortran like variables in order to
     query about all their Fortran properties.
   }];
 
   let methods = [
     InterfaceMethod<
       /*desc=*/"Get the address produced by the definition",
bool isBox() {
       return fir::unwrapRefType(getBase().getType()).isa<fir::BaseBoxType>();
     }
 
     /// Interface verifier imlementation.
     mlir::LogicalResult verifyImpl();
 
   }];
 
+  let cppNamespace = "fir";
+
   let verify = [{
     return ::mlir::cast<::fir::FortranVariableOpInterface>($_op).verifyImpl();
   }];
 }
 
 #endif  // FORTRANVARIABLEINTERFACE

flang/lib/Optimizer/Dialect/FortranVariableInterface.cpp

 //===-- FortranVariableInterface.cpp.cpp ----------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Coding style: https://mlir.llvm.org/getting_started/DeveloperGuide/
 //
 //===----------------------------------------------------------------------===//
 
 #include "flang/Optimizer/Dialect/FortranVariableInterface.h"
 
-namespace fir {
 #include "flang/Optimizer/Dialect/FortranVariableInterface.cpp.inc"
-}
 
 mlir::LogicalResult fir::FortranVariableOpInterface::verifyImpl() {
   const unsigned numExplicitTypeParams = getExplicitTypeParams().size();
   if (isCharacter()) {
     if (numExplicitTypeParams > 1)
       return emitOpError(
           "of character entity must have at most one length parameter");
     if (numExplicitTypeParams == 0 && !isBox())