[patch] libstdc++/66059 optimise std::make_integer_sequence

[Jonathan Wakely]

I've been talking about a compiler built-in to implement
make_integer_sequence since before the proposal even made it into the
standard, so I tried to implement one that would allow:

template<typename _Tp, _Tp _Num>
  using make_integer_sequence = integer_sequence< __intseq(_Tp, _Num) >;

But I don't know the front-end well enough to make that work.

Instead here's a much more efficient implementation that takes a
divide-and-conquer approach rather than building the sequence
linearly.

Tested powerpc64le-linux, committed to trunk.


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