Extracted Dsymbol.hasPointers to visitor in dsymbolsem

[MatthewQiu-5]

Part of the project to separate semantic routines from AST nodes: https://github.com/dlang/project-ideas/blob/master/separate-semantic-routines-from-ast-nodes/description.md

@RazvanN7

[dlang-bot]

Thanks for your pull request and interest in making D better, @MatthewQiu-5! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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dub run digger -- build "master + dmd#21140"

[thewilsonator]

This is used by LDC so it actually needs to remain extern(C++) despite what Iain said. When you add this back to the C++ interface, add it to the dmd namespace, not the global one.

[ibuclaw]

@thewilsonator this is correct. top-level symbol is extern(D). The exported C++ symbol is in the dmd namespace in cxxfrontend.d.

Used by LDC, required to be in the C++ interface

Symbol in dmd namespace

[thewilsonator]

This has merge conflicts, please rebase.

[thewilsonator]

Apart from the style comments from Iain and the merge conflict, this looks good.