Inconsistent behavior of `IsAnonymousRecordDecl()` with `isCpp`

[MeteorsLiu]

When the isCpp is enabled (true), the IsAnonymousRecordDecl() method correctly identifies anonymous structs. However, when isCpp is disabled (false), this method fails to recognize anonymous structs properly.

This inconsistency causes the subsequent logic to generate an AST node with an identifier "unname"(likely unintended), as the anonymous struct isn't handled at the expected stage.

The AST we expected, it works well when isCpp is true

{
  "Doc": null,
  "Loc": {
    "File": "testdata/struct/temp.h",
    "_Type": "Location"
  },
  "Name": null,
  "Parent": null,
  ...
}

Unexpected, when isCpp is false

{
  "Doc": null,
  "Loc": {
    "File": "testdata/struct/temp.h",
    "_Type": "Location"
  },
  "Name": {
    "Name": "struct (unnamed at testdata/struct/temp.h:1:1)",
    "_Type": "Ident"
  },
  "Parent": null,
  ...
}