Add test for format args capture bug.

Author: m-ou-se

src/test/ui/fmt/format-args-capture-issue-93378.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let a = "a";
+    let b = "b";
+
+    println!("{a} {b} {} {} {c} {}", c = "c");
+    //~^ ERROR: invalid reference to positional arguments 1 and 2 (there is 1 argument)
+}

src/test/ui/fmt/format-args-capture-issue-93378.stderr

@@ -0,0 +1,10 @@
+error: invalid reference to positional arguments 1 and 2 (there is 1 argument)
+  --> $DIR/format-args-capture-issue-93378.rs:5:26
+   |
+LL |     println!("{a} {b} {} {} {c} {}", c = "c");
+   |                          ^^     ^^
+   |
+   = note: positional arguments are zero-based
+
+error: aborting due to previous error
+