Add flag to configure large_assignments lint

The `large_assignments` lints detects moves over specified limit.  The
limit is configured through `move_size_limit = "N"` attribute placed at
the root of a crate. When attribute is absent, the lint is disabled.

Make it possible to enable the lint without making any changes to the
source code, through a new flag `-Zmove-size-limit=N`.  For example, to
detect moves exceeding 1023 bytes in a cargo crate, including all
dependencies one could use:

```
$ env RUSTFLAGS=-Zmove-size-limit=1024 cargo build -vv
```

compiler/rustc_interface/src/tests.rs

@@ -720,6 +720,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(merge_functions, Some(MergeFunctions::Disabled));
     tracked!(mir_emit_retag, true);
     tracked!(mir_opt_level, Some(4));
+    tracked!(move_size_limit, Some(4096));
     tracked!(mutable_noalias, Some(true));
     tracked!(new_llvm_pass_manager, Some(true));
     tracked!(no_generate_arange_section, true);