MIR Lower - Properly construct+drop structs created and then immediately dropped
MIR Lower - Properly construct+drop structs created and then immediately dropped

E.g.
```rust
struct Foo(i32);
Foo(123);
```

thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc