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

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

> Let's just increase to 9 bits.
> 
> Have you checked whether the size of `Stmt` or `Expr` changes when you do this?

I changed it to 9 bits and checked that `Stmt` is still 8 bytes and `Expr` is still 16 bytes.

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