evaluate obligations in LIFO order during closure projection

This is an annoying gotcha with the projection cache's handling of
nested obligations.

Nested projection obligations enter the issue in this case:
```
DEBUG:rustc::traits::project: AssociatedTypeNormalizer: depth=3
normalized
<std::iter::Map<std::ops::Range<i32>,
[closure@not-a-recursion-error.rs:5:30: 5:53]> as
std::iter::IntoIterator>::Item to _#7t with 12 add'l obligations
```

Here the normalization result is the result of the nested impl
`<[closure@not-a-recursion-error.rs:5:30: 5:53] as FnMut(i32)>::Output`,
which is an additional obligation that is a part of "add'l obligations".

By itself, this is proper behaviour - the additional obligation is
returned, and the RFC 447 rules ensure that it is processed before the
output `#_7t` is used in any way.

However, the projection cache breaks this - it caches the
`<std::iter::Map<std::ops::Range<i32>,[closure@not-a-recursion-error.rs:5:30:
5:53]> as std::iter::IntoIterator>::Item = #_7t` resolution. Now
everybody else that attempts to look up the projection will just get
`#_7t` *without* any additional obligations. This obviously causes all
sorts of trouble (here a spurious `EvaluatedToAmbig` results in
specializations not being discarded
[here](https://github.com/rust-lang/rust/blob/9ca50bd4d50b55456e88a8c3ad8fcc9798f57522/src/librustc/traits/select.rs#L1705)).

The compiler works even with this projection cache gotcha because in most
cases during "one-pass evaluation". we tend to process obligations in LIFO
order - after an obligation is added to the cache, we process its nested
obligations before we do anything else (and if we have a cycle, we handle
it specifically) - which makes sure the inference variables are resolved
before they are used.

That "LIFO" order That was not done when projecting out of a closure, so
let's just fix that for the time being.

Fixes #38033.

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