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

Summary:
After 60573ae6 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.

Reviewers: rdhindsa

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71222