ir: context: fall back to use the declaration as a key when there's no USR.

Apparently MSVC isn't that good at giving us USRs...

Fixes #271

Author: emilio

libbindgen/src/ir/context.rs

@@ -222,7 +222,7 @@ impl<'ctx> BindgenContext<'ctx> {
                 error!("Valid declaration with no USR: {:?}, {:?}",
                        declaration,
                        location);
-                return;
+                TypeKey::Declaration(declaration)
             };
 
             let old = self.types.insert(key, id);

libbindgen/tests/expectations/tests/msvc-no-usr.rs

@@ -0,0 +1,19 @@
+/* automatically generated by rust-bindgen */
+
+
+#![allow(non_snake_case)]
+
+
+#[repr(C)]
+#[derive(Debug, Copy)]
+pub struct A {
+    pub foo: usize,
+}
+#[test]
+fn bindgen_test_layout_A() {
+    assert_eq!(::std::mem::size_of::<A>() , 4usize);
+    assert_eq!(::std::mem::align_of::<A>() , 4usize);
+}
+impl Clone for A {
+    fn clone(&self) -> Self { *self }
+}

libbindgen/tests/headers/msvc-no-usr.hpp

@@ -0,0 +1,8 @@
+
+typedef unsigned size_t;
+
+class A {
+  const size_t foo;
+
+  A() : foo(5) {}
+};