fix template union forward declaration, close rust-lang#1768

Author: Danielmelody

Diff for: bindgen-tests/tests/expectations/tests/union_template_forward_decl.rs

@@ -0,0 +1,17 @@
+// bindgen-flags: --with-derive-hash --with-derive-partialeq --with-derive-eq
+//
+template <typename value_t>
+union declare_union; // Primary template declared, but never defined.
+
+template <typename value_t> union declare_union<value_t *> {
+  declare_union() {}
+  declare_union(value_t *a_value) : value(a_value) {}
+  value_t *value;
+};
+
+template <typename value_t> union define_union {
+  define_union() {}
+  define_union(value_t *a_value) : value(a_value) {}
+  value_t *value;
+  int dummy;
+};

Diff for: bindgen-tests/tests/headers/union_template_forward_decl.hpp

@@ -1275,6 +1275,7 @@ impl CompInfo {
                 CXCursor_ParmDecl => true,
                 CXCursor_StructDecl | CXCursor_UnionDecl |
                 CXCursor_ClassDecl => !cur.is_definition(),
+                CXCursor_ClassTemplate => kind == CompKind::Union && !cur.is_definition(), 
                 _ => false,
             });