[clang] [clang][NFC] Increase NumStmtBits by 2 as we are approaching the limit (PR #120341)

[John McCall via cfe-commits]

https://github.com/rjmccall commented:

Let's just increase to 9 bits.

Have you checked whether the size of `Stmt` or `Expr` changes when you do this?

https://github.com/llvm/llvm-project/pull/120341