Redefine __traits(getPointerBitmap, S) for types without pointers to return size only

[JohanEngelen]

This greatly reduces compile time for large types without pointers, because no need to create a large array literal with zeros.

See DMD and druntime implementation here: dlang/dmd#22289

[dlang-bot]

Thanks for your pull request and interest in making D better, @JohanEngelen! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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