Drop temporaries created in a condition, even if it's a let chain
Fixes#100513.
During the lowering from AST to HIR we wrap expressions acting as conditions in a `DropTemps` expression so that any temporaries created in the condition are dropped after the condition is executed. Effectively this means we transform
```rust
if Some(1).is_some() { .. }
```
into (roughly)
```rust
if { let _t = Some(1).is_some(); _t } { .. }
```
so that if we create any temporaries, they're lifted into the new scope surrounding the condition, so for example something along the lines of
```rust
if { let temp = Some(1); let _t = temp.is_some(); _t }.
```
Before this PR, if the condition contained any let expressions we would not introduce that new scope, instead leaving the condition alone. This meant that in a let-chain like
```rust
if get_drop("first").is_some() && let None = get_drop("last") {
println!("second");
} else { .. }
```
the temporary created for `get_drop("first")` would be lifted into the _surrounding block_, which caused it to be dropped after the execution of the entire `if` expression.
After this PR, we wrap everything but the `let` expression in terminating scopes. The upside to this solution is that it's minimally invasive, but the downside is that in the worst case, an expression with `let` exprs interspersed like
```rust
if get_drop("first").is_some()
&& let Some(_a) = get_drop("fifth")
&& get_drop("second").is_some()
&& let Some(_b) = get_drop("fourth") { .. }
```
gets _multiple_ new scopes, roughly
```rust
if { let _t = get_drop("first").is_some(); _t }
&& let Some(_a) = get_drop("fifth")
&& { let _t = get_drop("second").is_some(); _t }
&& let Some(_b) = get_drop("fourth") { .. }
```
so instead of all of the temporaries being dropped at the end of the entire condition, they will be dropped right after they're evaluated (before the subsequent `let` expr). So while I'd say the drop behavior around let-chains is _less_ surprising after this PR, it still might not exactly match what people might expect.
For tests, I've just extended the drop order tests added in #100526. I'm not sure if that's the best way to go about it, though, so suggestions are welcome.
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