Drop partially bound function parameters in the expected order
Given the function
```rust
fn foo((_x, _): (LogDrop, LogDrop), (_, _y): (LogDrop, LogDrop)) {}
```
Prior to 1.12.0 we dropped both `_x` and `_y` before the rest of their
respective parameters, since then we dropped `_x` and `_y` after. The
original order appears to be the correct order, as the value created
later is dropped first, so we revert to that order and add a test for
it.
While this is technically a breaking change, I can't work out how
anyone could be relying on this without making their code very
brittle. If this is considered to be too likely to break real world code
then I can revert the change and change the test to check for the
current order.
c4b8735872