tests: add regression test for incorrect BytePos manipulation triggering assertion

Issue: <https://github.com/rust-lang/rust/issues/128717>

tests/ui/typeck/suggest-arg-comma-delete-ice.rs

@@ -0,0 +1,19 @@
+//! Previously, we tried to remove extra arg commas when providing extra arg removal suggestions.
+//! One of the edge cases is having to account for an arg that has a closing delimiter `)`
+//! following it. However, the previous suggestion code assumed that the delimiter is in fact
+//! exactly the 1-byte `)` character. This assumption was proven incorrect, because we recover
+//! from Unicode-confusable delimiters in the parser, which means that the ending delimiter could be
+//! a multi-byte codepoint that looks *like* a `)`. Subtracing 1 byte could land us in the middle of
+//! a codepoint, triggering a codepoint boundary assertion.
+//!
+//! issue: rust-lang/rust#128717
+
+fn main() {
+    // The following example has been modified from #128717 to remove irrelevant Unicode as they do
+    // not otherwise partake in the right delimiter calculation causing the codepoint boundary
+    // assertion.
+    main(rahh）;
+    //~^ ERROR unknown start of token
+    //~| ERROR this function takes 0 arguments but 1 argument was supplied
+    //~| ERROR cannot find value `rahh` in this scope
+}

tests/ui/typeck/suggest-arg-comma-delete-ice.stderr

@@ -0,0 +1,38 @@
+error: unknown start of token: \u{ff09}
+  --> $DIR/suggest-arg-comma-delete-ice.rs:15:14
+   |
+LL |     main(rahh）;
+   |              ^^
+   |
+help: Unicode character '）' (Fullwidth Right Parenthesis) looks like ')' (Right Parenthesis), but it is not
+   |
+LL |     main(rahh);
+   |              ~
+
+error[E0425]: cannot find value `rahh` in this scope
+  --> $DIR/suggest-arg-comma-delete-ice.rs:15:10
+   |
+LL |     main(rahh）;
+   |          ^^^^ not found in this scope
+
+error[E0061]: this function takes 0 arguments but 1 argument was supplied
+  --> $DIR/suggest-arg-comma-delete-ice.rs:15:5
+   |
+LL |     main(rahh）;
+   |     ^^^^ ---- unexpected argument
+   |
+note: function defined here
+  --> $DIR/suggest-arg-comma-delete-ice.rs:11:4
+   |
+LL | fn main() {
+   |    ^^^^
+help: remove the extra argument
+   |
+LL -     main(rahh）;
+LL +     main(）;
+   |
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0061, E0425.
+For more information about an error, try `rustc --explain E0061`.