MSVC Compatibility: [[msvc::no_unique_address]] doesn't work in a base class

[mingxwa]

struct A {};

struct B {
  [[msvc::no_unique_address]] A val;
};

struct C : B {
  char x;
};

static_assert(sizeof(C) == 1);  // clang-cl thinks sizeof(C) is 2, while it should be 1

When the standard [[no_unique_address]] is used in regular mode, the memory layout of C is expected.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mingxin Wang (mingxwa)

```cpp struct A {};

struct B {
[[msvc::no_unique_address]] A val;
};

struct C : B {
char x;
};

static_assert(sizeof(C) == 1); // clang-cl thinks sizeof(C) is 2, while it should be 1

When the standard `[[no_unique_address]]` is used in regular mode, the memory layout of `C` is expected.
</details>

[shafik]

CC @AaronBallman

[cor3ntin]

MSVC do think this is well formed, so this is a bug https://compiler-explorer.com/z/3rGeEnK3x