tests: add regression test for #128845

For codepoint boundary assertion triggered by a let stmt compound
assignment removal suggestion when encountering recovered multi-byte
compound ops.

Issue: <#128845>

tests/ui/parser/suggest-remove-compount-assign-let-ice.rs

@@ -0,0 +1,16 @@
+//! Previously we would try to issue a suggestion for `let x <op>= 1`, i.e. a compound assignment
+//! within a `let` binding, to remove the `<op>`. The suggestion code unfortunately incorrectly
+//! assumed that the `<op>` is an exactly-1-byte ASCII character, but this assumption is incorrect
+//! because we also recover Unicode-confusables like `➖=` as `-=`. In this example, the suggestion
+//! code used a `+ BytePos(1)` to calculate the span of the `<op>` codepoint that looks like `-` but
+//! the mult-byte Unicode look-alike would cause the suggested removal span to be inside a
+//! multi-byte codepoint boundary, triggering a codepoint boundary assertion.
+//!
+//! issue: rust-lang/rust#128845
+
+fn main() {
+    // Adapted from #128845 but with irrelevant components removed and simplified.
+    let x ➖= 1;
+    //~^ ERROR unknown start of token: \u{2796}
+    //~| ERROR: can't reassign to an uninitialized variable
+}

tests/ui/parser/suggest-remove-compount-assign-let-ice.stderr

@@ -0,0 +1,26 @@
+error: unknown start of token: \u{2796}
+  --> $DIR/suggest-remove-compount-assign-let-ice.rs:13:11
+   |
+LL |     let x ➖= 1;
+   |           ^^
+   |
+help: Unicode character '➖' (Heavy Minus Sign) looks like '-' (Minus/Hyphen), but it is not
+   |
+LL |     let x -= 1;
+   |           ~
+
+error: can't reassign to an uninitialized variable
+  --> $DIR/suggest-remove-compount-assign-let-ice.rs:13:11
+   |
+LL |     let x ➖= 1;
+   |           ^^^
+   |
+   = help: if you meant to overwrite, remove the `let` binding
+help: initialize the variable
+   |
+LL -     let x ➖= 1;
+LL +     let x = 1;
+   |
+
+error: aborting due to 2 previous errors
+