Refactor ForStmt: having it contain a StmtBlock instead of subclassing

StmtBlock.  This is more consistent with IfStmt and also conceptually makes
more sense - a forstmt "isn't" its body, it contains its body.

This is step 1/N towards merging BasicBlock and StmtBlock.  This is required
because in the new regime StmtBlock will have a use list (just like BasicBlock
does) of operands, and ForStmt already has a use list for its induction
variable.

This is a mechanical patch, NFC.

PiperOrigin-RevId: 226684158