[clang-tidy] Treat fields in anonymous records as names in enclosing scope...

[clang-tidy] Treat fields in anonymous records as names in enclosing scope when checking name styles (#75701)

Currently, fields in anonymous records are treated as normal record
members during naming style check. This can be undesirable in certain
situations since these fields are used just like names in their
enclosing scopes:

```c++
class Foo {
  union {
    int iv_;    // warning: invalid case style for public member 'iv_'
    float fv_;  // warning: invalid case style for public member 'fv_'
  };
};
```

`iv_` and `fv_` are used in the code like private members of `Foo` but
their naming style comes from rules for public members.

This PR changes this behavior. It adds a new option
`CheckAnonFieldInParent` to `readability-identifier-naming`. When set to
`true`, fields in anonymous records will be treated as names in their
enclosing scopes when checking name styles. Specifically:

- If the anonymous record is defined within the file scope or in a
namespace scope, treat its fields as global variables when checking name
styles;
- If the anonymous record is defined within a function, treat its fields
as local variables when checking name styles;
- If the anonymous record is defined within a non-anonymous record,
treat its fields as non-static record members when checking name styles.