[HLSL] Constant buffer codegen crashes when `cbuffer` contains a class

[hekota]

class Foo {
  float x;
};

cbuffer cb {
    Foo foo;
}
RWBuffer<float> Buf : register(u0);

[numthreads(4,1,1)]
void main() {
      Buf[0] = foo.x;
}

https://godbolt.org/z/c1fYheWWb

The codegen crashes in clang::CodeGen::HLSLBufferLayoutBuilder::layoutField because the layout code is not handling classes, only structs.

[llvmbot]

@llvm/issue-subscribers-bug

Author: Helena Kotas (hekota)

``` class Foo { float x; };

cbuffer cb {
Foo foo;
}
RWBuffer<float> Buf : register(u0);

[numthreads(4,1,1)]
void main() {
Buf[0] = foo.x;
}

https://godbolt.org/z/K87MTMzq5

The codegen crashes in `clang::CodeGen::HLSLBufferLayoutBuilder::layoutField` because the layout code is not handling classes, only structs.
</details>

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Helena Kotas (hekota)

``` class Foo { float x; };

cbuffer cb {
Foo foo;
}
RWBuffer<float> Buf : register(u0);

[numthreads(4,1,1)]
void main() {
Buf[0] = foo.x;
}

https://godbolt.org/z/c1fYheWWb

The codegen crashes in `clang::CodeGen::HLSLBufferLayoutBuilder::layoutField` because the layout code is not handling classes, only structs.
</details>