6731: Add replace_match_with_if_let assist r=matklad a=Veykril
Basically the counterpart to `replace_if_let_with_match`, I personally sometimes want to replace matches like
```rust
match foo {
pat => expr,
_ => (),
}
```
into the corresponding
```rust
if let pat = foo {
expr
}
```
which is the main reasoning behind this.
I put this into the same file as `replace_if_let_with_match` because the are complementing each other and I would probably rename the file to something like `replace_if_let_match` but I didn't do that for now because I was unsure whether git would still view this as a rename or not due to the amount of changes in the file so that the diff is still properly visible for now.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
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