ir: Deal with name conflicts correctly in declaration type resolution.

So, basically, make opaque items last, and make previous names override
them.

Fixes #649

tests/rust/decl_name_conflicting.rs

@@ -0,0 +1,14 @@
+mod uhoh {
+    enum BindingType { Buffer, NotBuffer }
+}
+
+#[repr(u32)]
+pub enum BindingType { Buffer = 0, NotBuffer = 1 }
+
+#[repr(C)]
+pub struct BindGroupLayoutEntry {
+    pub ty: BindingType, // This is the repr(u32) enum
+}
+
+#[no_mangle]
+pub extern "C" fn root(entry: BindGroupLayoutEntry) {}