Extracted Dsymbol.oneMember to a visitor in dsymbolsem

[MatthewQiu-5]

Part of the project to separate semantic routines from AST nodes. @RazvanN7

[dlang-bot]

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[thewilsonator]

This otherwise looks good

[thewilsonator]

This is unused by LDC/GDC remove the extern(C++), this is causing CircleCI to fail because the generated frontend.h doesn't match.

[MatthewQiu-5]

Understood, I'll go ahead and remove extern(C++).