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

test/cases/compile_errors/reassign_to_array_parameter.zig

@@ -0,0 +1,12 @@
+fn reassign(a: [3]f32) void {
+    a = [3]f32{4, 5, 6};
+}
+export fn entry() void {
+    reassign(.{1, 2, 3});
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:15: error: cannot assign to constant