Refactor MLFunction to contain a StmtBlock for its body instead of inheriting

from it.  This is necessary progress to squaring away the parent relationship
that a StmtBlock has with its enclosing if/for/fn, and makes room for functions
to have more than one block in the future.  This also removes IfClause and ForStmtBody.

This is step 5/n towards merging instructions and statements, NFC.

PiperOrigin-RevId: 226936541