Rust: In 'type_for_mode' langhook also consider all 'int_n' modes/types

As reported in "Rust related issues" at the end of
<https://inbox.sourceware.org/gcc-patches/20221219212344.ewtuzd2jtztm77x2@lug-owl.de>
"Modula-2 / Rust: Many targets failing",
<Rust-GCC/gccrs#1713>
"Test failure on msp430-elfbare target", for '--target=msp430-elfbar' we ICE:

    <built-in>: internal compiler error: Segmentation fault
    0xf2efbf crash_signal
          ../../gcc/gcc/toplev.cc:314
    0x120c8c7 build_function_type(tree_node*, tree_node*, bool)
          ../../gcc/gcc/tree.cc:7360
    0x120cc20 build_function_type_list(tree_node*, ...)
          ../../gcc/gcc/tree.cc:7442
    0x120d16b build_common_builtin_nodes()
          ../../gcc/gcc/tree.cc:9883
    0x8449b4 grs_langhook_init
          ../../gcc/gcc/rust/rust-lang.cc:132
    [...]

This is due to 'tmp == NULL' for 'gcc/tree.cc:build_common_builtin_nodes':

    tmp = lang_hooks.types.type_for_mode (targetm.eh_return_filter_mode (), 0);

..., were 'targetm.eh_return_filter_mode' (that is,
'gcc/targhooks.cc:default_eh_return_filter_mode',
via calling 'targetm.unwind_word_mode':
'gcc/config/msp430/msp430.cc:msp430_unwind_word_mode') returns 'PSImode',
and 'lang_hooks.types.type_for_mode' (that is,
'gcc/rust/rust-lang.cc:grs_langhook_type_for_mode') returns 'NULL' for
'PSImode'.

As, for example, discussed in <https://gcc.gnu.org/PR46805>
"ICE: SIGSEGV in optab_for_tree_code (optabs.c:407) with -O -fno-tree-scev-cprop -ftree-vectorize",
we have to support "random" modes/types in the 'type_for_mode' langhook.

	gcc/rust/
	* rust-lang.cc (grs_langhook_type_for_mode): Also consider all
	'int_n' modes/types.

gcc/rust/rust-lang.cc

@@ -231,6 +231,21 @@ grs_langhook_type_for_mode (machine_mode mode, int unsignedp)
       if (mode == TYPE_MODE (complex_integer_type_node) && !unsignedp)
 	return complex_integer_type_node;
     }
+
+  /* See (a) <https://github.com/Rust-GCC/gccrs/issues/1713>
+     "Test failure on msp430-elfbare target", and
+     (b) <https://gcc.gnu.org/PR46805>
+     "ICE: SIGSEGV in optab_for_tree_code (optabs.c:407) with -O -fno-tree-scev-cprop -ftree-vectorize"
+     -- we have to support "random" modes/types here.
+     TODO Clean all this up (either locally, or preferably per PR46805:
+     "Ideally we'd never use lang_hooks.types.type_for_mode (or _for_size) in the
+     middle-end but had a pure middle-end based implementation".  */
+  for (size_t i = 0; i < NUM_INT_N_ENTS; i ++)
+    if (int_n_enabled_p[i]
+	&& mode == int_n_data[i].m)
+      return (unsignedp ? int_n_trees[i].unsigned_type
+	      : int_n_trees[i].signed_type);
+
   /* gcc_unreachable */
   return NULL;
 }