Sema: add source location to coerce result ptr, fix negation error

test/cases/compile_errors/reassign_to_struct_parameter.zig

@@ -0,0 +1,15 @@
+const S = struct {
+    x: u32,
+};
+fn reassign(s: S) void {
+    s = S{.x = 2};
+}
+export fn entry() void {
+    reassign(S{.x = 3});
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :5:10: error: cannot assign to constant