[mlir][EmitC] Remove Pure trait from `emitc.include`

[christopherbate]

Summary

The op emitc.include does not have results and thus will be elided
during canonicalization, which is not correct behavior. This change
removes the 'Pure' trait and adds a canonicalization test.

[christopherbate]

Fix commit message.

[jpienaar]

LG, adding Marius for visibility.

[marbre]

Thanks for adding me @jpienaar and thanks @christopherbate for improving the EmitC dialect! LGTM.

mlir/include/mlir/Dialect/EmitC/IR/EmitC.td

def EmitC_ConstantOp : EmitC_Op<"constant", [ConstantLike]> {
   let results = (outs AnyType);
 
   let hasFolder = 1;
   // TODO: Disallow empty constants.
   let hasVerifier = 1;
 }
 
 def EmitC_IncludeOp
-    : EmitC_Op<"include", [Pure, HasParent<"ModuleOp">]> {
+    : EmitC_Op<"include", [HasParent<"ModuleOp">]> {
   let summary = "Include operation";
   let description = [{
     The `include` operation allows to define a source file inclusion via the
     `#include` directive.
 
     Example:
 
     ```mlir

mlir/test/Dialect/EmitC/ops.mlir

 // RUN: mlir-opt %s | mlir-opt | FileCheck %s
+// RUN: mlir-opt %s -canonicalize | FileCheck %s
 
+// CHECK: emitc.include <"test.h">
+// CHECK: emitc.include "test.h"
 emitc.include <"test.h">
 emitc.include "test.h"
 
 // CHECK-LABEL: func @f(%{{.*}}: i32, %{{.*}}: !emitc.opaque<"int32_t">) {
 func.func @f(%arg0: i32, %f: !emitc.opaque<"int32_t">) {
   %1 = "emitc.call"() {callee = "blah"} : () -> i64
   emitc.call "foo" (%1) {args = [
     0 : index, dense<[0, 1]> : tensor<2xi32>, 0 : index