Improve error message for `||` (or) in let chains
**Description**
This PR improves the error message when using `||` in an if let chain expression, addressing #140263.
**Changes**
1. Creates a dedicated error message specifically for `||` usage in let chains
2. Points the primary span directly at the `||` operator
3. Removes confusing secondary notes about "let statements" and unsupported contexts
5. Adds UI tests verifying the new error message and valid cases
**Before**
```rust
error: expected expression, found let statement
--> src/main.rs:2:8
|
2 | if let true = true || false {}
| ^^^^^^^^^^^^^^^
|
= note: only supported directly in conditions of if and while expressions
note: || operators are not supported in let chain expressions
--> src/main.rs:2:24
|
2 | if let true = true || false {}
|
```
**After**
```rust
error: `||` operators are not supported in let chain conditions
--> src/main.rs:2:24
|
2 | if let true = true || false {}
| ^^
```
**Implementation details**
1. Added new `OrInLetChain` diagnostic in errors.rs
2. Modified `CondChecker` in expr.rs to prioritize the `||` error
3. Updated fluent message definitions to use clearer wording
**Related issue**
Fixes#140263
cc ```@ehuss``` (issue author)
6f6fa0f23a