Revert "Replace calls to CFStringCreateWithCString with constant CFStrings"

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Reverts #4694

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@swift-ci please test

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Verified locally, this does repair the regression. CC: @jckarter

s10Foundation19_NSCFConstantStringCN => "type metadata for Foundation._NSCFConstantString"

_NSCFConstantString is defined as internal final: https://github.com/apple/swift-corelibs-foundation/blob/main/Sources/Foundation/NSCFString.swift#L62

This means that the type metadata for that is not exported, and there are now references to that in FoundationXML which cannot be fulfilled as the symbol is not visible outside of Foundation. This is happening as the CFXMLInterface.c file was modified to use CFSTR("") which generates the CFString literal which will require the reference to the type metadata. I don't think that this is worth making _NSCFConstantString public.

CC: @parkera