Fix specialization of SomeType<N> when N is a const parameter.

Fixes #761.

tests/rust/const_generics_thru.rs

@@ -0,0 +1,22 @@
+// Propagating const arguments through generics that use generics.
+
+#[repr(C)]
+pub struct Inner<const N: usize> {
+    pub bytes: [u8; N],
+}
+
+#[repr(C)]
+pub struct Outer<const N: usize> {
+    pub inner: Inner<N>, // don't declare two different structs named `Inner_N`
+}
+
+#[no_mangle]
+pub extern "C" fn one() -> Outer<1> {
+    Outer { inner: Inner { bytes: [0] } }
+}
+
+#[no_mangle]
+pub extern "C" fn two() -> Outer<2> {
+    Outer { inner: Inner { bytes: [0, 0] } }
+}
+