Move is_specialisation_of to another header file

[Frederick Virchanza Gotham]

On Thu, Jan 8, 2026 at 3:04 PM Jonathan Wakely wrote:
>>
> > You'll see I also have "__is_specialization_of_as_struct" in order to
> > accommodate pre-C++14.
>
> That isn't necessary because nothing in libstdc++ needs it before
> C++14, and so it just slows down compilation for no reason.


I have amended my patch as follows:

    https://github.com/healytpk/gcc-thomas-healy/commit/tag_header_is_specialization_of

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