Include Stmt.h where it seems to be necessary for modules builds

[rnk]

Summary

After 60573ae6fe50 removed an include of Expr.h from ASTContext.h, this
header fails to compile in some modular build configurations. I have not
been able to reproduce the problem locally. The header compiles fine in
isolation. However, based on reading the code, it seems like it would
require Stmt to be complete. Based on that intuition, we decided to add the
include.

[rdhindsa]

LGTM

clang/include/clang/Tooling/Refactoring/ASTSelection.h

 //===--- ASTSelection.h - Clang refactoring library -----------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLING_REFACTOR_AST_SELECTION_H
 #define LLVM_CLANG_TOOLING_REFACTOR_AST_SELECTION_H
 
 #include "clang/AST/ASTTypeTraits.h"
+#include "clang/AST/Stmt.h"
 #include "clang/Basic/LLVM.h"
 #include "clang/Basic/SourceLocation.h"
 #include <vector>
 
 namespace clang {
 
 class ASTContext;