ir: Handle NonZero and simplify Option<NonZero> like we simplify Option<NonNull>.

Fixes #646

tests/rust/nonzero.rs

@@ -0,0 +1,30 @@
+use std::num::*;
+
+#[repr(C)]
+pub struct NonZeroTest {
+    pub a: NonZeroU8,
+    pub b: NonZeroU16,
+    pub c: NonZeroU32,
+    pub d: NonZeroU64,
+    pub e: NonZeroI8,
+    pub f: NonZeroI16,
+    pub g: NonZeroI32,
+    pub h: NonZeroI64,
+    pub i: Option<NonZeroI64>,
+    pub j: *const Option<Option<NonZeroI64>>,
+}
+
+#[no_mangle]
+pub extern "C" fn root(
+    test: NonZeroTest,
+    a: NonZeroU8,
+    b: NonZeroU16,
+    c: NonZeroU32,
+    d: NonZeroU64,
+    e: NonZeroI8,
+    f: NonZeroI16,
+    g: NonZeroI32,
+    h: NonZeroI64,
+    i: Option<NonZeroI64>,
+    j: *const Option<Option<NonZeroI64>>,
+) {}

tests/rust/nonzero.toml

@@ -0,0 +1,13 @@
+header = """
+#if 0
+''' '
+#endif
+
+#ifdef __cplusplus
+struct NonZeroI64;
+#endif
+
+#if 0
+' '''
+#endif
+"""