Don't produce debug information for compiler-introduced-vars when desugaring assignments.

An assignment such as

(a, b) = (b, c);

desugars to the HIR

{ let (lhs, lhs) = (b, c); a = lhs; b = lhs; };

The repeated `lhs` leads to multiple Locals assigned to the same DILocalVariable. Rather than
attempting to fix that, get rid of the debug info for these bindings that don't even exist
in the program to begin with.

Fixes #138198

tests/codegen/assign-desugar-debuginfo.rs

@@ -0,0 +1,18 @@
+//@ compile-flags: -g -Zmir-opt-level=0
+
+#![crate_type = "lib"]
+
+#[inline(never)]
+fn swizzle(a: u32, b: u32, c: u32) -> (u32, (u32, u32)) {
+    (b, (c, a))
+}
+
+pub fn work() {
+    let mut a = 1;
+    let mut b = 2;
+    let mut c = 3;
+    (a, (b, c)) = swizzle(a, b, c);
+    println!("{a} {b} {c}");
+}
+
+// CHECK-NOT: !DILocalVariable(name: "lhs",