[clang] Implement CWG1693 to allow empty declarations in classes.

[keinflue]

Clang rejects the following with -pedantic-errors in C++ (https://godbolt.org/z/jbaPKMqhs):

union X{;};

<source>:1:9: error: extra ';' inside a union [-Werror,-Wextra-semi]
    1 | union X{;};
      |         ^

However, I do not see anything prohibiting empty declarations in unions in general. They are only prohibited in anonymous unions.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (keinflue)

Clang rejects the following with `-pedantic-errors` in C++ (https://godbolt.org/z/jbaPKMqhs):

union X{;};

<source>:1:9: error: extra ';' inside a union [-Werror,-Wextra-semi]
    1 | union X{;};
      |         ^

However, I do not see anything prohibiting empty declarations in unions in general. They are only prohibited in anonymous unions.

[keinflue]

I just realized that clang rejects empty declarations in all classes with -pedantic-errors. I assume that CWG 1693 was never implemented.