Wrong layout for generic union when not used untagged unions.

Input C/C++ Header

enum class StyleTimingKeyword : unsigned char {
  Foo,
  Bar,
};

enum class StyleStepPosition : unsigned char {
  Baz,
  Bar,
};

template<typename Integer, typename Number>
union StyleTimingFunction {
  enum class Tag : unsigned char {
    Keyword,
    CubicBezier,
    Steps
  };

  struct Keyword_Body {
    Tag tag;
    StyleTimingKeyword _0;
  };

  struct CubicBezier_Body {
    Tag tag;
    Number x1;
    Number y1;
    Number x2;
    Number y2;
  };

  struct Steps_Body {
    Tag tag;
    Integer _0;
    StyleStepPosition _1;
  };

  struct {
    Tag tag;
  };
  Keyword_Body keyword;
  CubicBezier_Body cubic_bezier;
  Steps_Body steps;
};

struct Foo {
  StyleTimingFunction<int, float> f;
};

Bindgen Invocation

$ ./target/debug/bindgen --rust-target 1.0 t.hpp -o out.rs
$ rustc --test out.rs
$ ./out

Actual Results

---- __bindgen_test_layout_StyleTimingFunction_open0_int_float_close0_instantiation stdout ----
thread '__bindgen_test_layout_StyleTimingFunction_open0_int_float_close0_instantiation' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `20`: Size of template specialization: StyleTimingFunction < :: std :: os :: raw :: c_int , f32 >', out.rs:131:5

The union has:

    pub bindgen_union_field: [u8; 0usize],

Because we cannot determine the struct layout at that point.

Expected Results

The test passes. We get this right with untagged unions.

Reproduce the issue using the C++ header shown in t.hpp, then run the bindgen invocation and rustc --test on the generated out.rs. Start with the generated layout assertion for StyleTimingFunction<int, float> and its bindgen_union_field; done means the generated test passes with the expected specialization size.