Do not use the canonical declaration for enum type building

Allow forward declarations to be used by avoiding the use of the
canonical declaration when building the underlying representation.

Author: dlrobertson

bindgen-tests/tests/expectations/tests/enum-forward-ref.rs

@@ -0,0 +1,19 @@
+// bindgen-flags: --default-enum-style=consts --constified-enum-module=Neg
+
+// A forward reference to an enum should work..
+enum Foo;
+enum Foo {
+  A
+};
+
+// A reference to an enum after the definition should work.
+enum Bar { B };
+enum Bar;
+
+// A forward reference to an enum in a function definition should work.
+enum Baz f(void);
+enum Baz { C };
+
+// A forward reference to an enum with no later definition should result in
+// a guess of the underlying type (i32).
+enum Qux;

bindgen-tests/tests/headers/enum-forward-ref.h

@@ -65,7 +65,10 @@ impl Enum {
             return Err(ParseError::Continue);
         }
 
-        let declaration = ty.declaration().canonical();
+        // Use the enum decl instead of the canonical destination. The
+        // cursor location of the first declaration may be the forward
+        // declaration.
+        let declaration = ty.declaration();
         let repr = declaration
             .enum_type()
             .and_then(|et| Item::from_ty(&et, declaration, None, ctx).ok());