[dclass.d] deduplicate logic for checking reserved classes

[thewilsonator]

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[thewilsonator]

dmd/compiler/src/dmd/dclass.d(239:13)[warn]: a virtual call inside a constructor may lead to unexpected results in the derived classes

I presume this is a fault with dscanner, that function is local. dlang-community/D-Scanner#968

[thewilsonator]

I don't know why this was originally commented out:

class TypeInfo_StaticArray {}
class TypeInfo_Array {}
void main() {}

yields

testtisa.d(2): Error: class `testtisa.TypeInfo_StaticArray` already exists at object.d(979). Perhaps in another function with the same name?
testtisa.d(3): Error: class `TypeInfo_Array` only object.d can define this reserved class name

and errors anyway.