[flang] Use `createOpWithBody` for section op, NFC (#74659)

Replace explicit calls to
```
  op = builder.create<SectionOp>(...)
  createBodyOfOp<SectionOp>(op, ...)
```
with a single call to
```
  createOpWithBody<SectionOp>(...)
```

This is NFC, that's what the `createOpWithBody` function does.