fold instead of obliterating args
Fixes#105608
we call `const_eval_resolve` on the following constant:
```
def: playground::{impl#0}::and::{constant#0},
substs: [
ConstKind::Unevaluated {
def: playground::{impl#0}::and::{constant#0},
substs: [
ConstKind::Value(0x0),
_,
]
}
_,
],
```
when expanded out to `ConstKind::Expr` there are no infer vars so we attempt to evaluate it after replacing infer vars with garbage, however the current logic for replacing with garbage replaces _the whole arg containing the infer var_ rather than just the infer var. This means that after garbage replacement has occured we attempt to evaluate:
```
def: playground::{impl#0}::and::{constant#0},
substs: [
PLACEHOLDER,
PLACEHOLDER,
],
```
Which then leads to ctfe being unable to evaluate the const. With this PR we attempt to evaluate:
```
def: playground::{impl#0}::and::{constant#0},
substs: [
ConstKind::Unevaluated {
def: playground::{impl#0}::and::{constant#0},
substs: [
ConstKind::Value(0x0),
PLACEHOLDER,
]
}
PLACEHOLDER,
],
```
which ctfe _can_ handle.
I am not entirely sure why this function is supposed to replace params with placeholders rather than just inference vars 🤔
01469693de