[clang] [clang][NFC] `#include "clang/AST/StmtVisitor.h"` where necessary (PR #172538)

[David Stone via cfe-commits]

davidstone wrote:

Just generally following the principle of include what you use. I was changing the contents of `StmtVisitor.h`, and it has one clear thing it exports, so I wanted to remove unused includes to potentially improve compile times and also include current accidental transitive includes to ensure future refactorings are easy.

https://github.com/llvm/llvm-project/pull/172538