Extracted Dsymbol.oneMember to a visitor in dsymbolsem

[MatthewQiu-5]

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

[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.

[dkorpel]

Suggested change

	* Determine if this symbol is only one.
	* Returns:
	* false, ps = null: There are 2 or more symbols
	* true, ps = null: There are zero symbols
	* true, ps = symbol: The one and only one symbol
	*/
	* Determine if this symbol is only one.
	* Returns:
	* false, ps = null: There are 2 or more symbols
	* true, ps = null: There are zero symbols
	* true, ps = symbol: The one and only one symbol
	*/