[NFC][AsmPrinter] Refactor constructVariableDIE (#66435)

Fold constructVariableDIEImpl into constructVariableDIE, simplify it and
group related functions.

Pull out the previously inline lambdas for visiting the active variant
of the DbgVariable to add location and related attributes as an overload
set for a private method
applyConcreteDbgVariableAttributes.

Rename applyVariableAttribute to reflect what kinds of attributes it
applies, and to contrast it with the new
applyConcreteDbgVariableAttributes.

Move constructLabelDIE down in the implementation file, so all of the
constructVariableDIE-related function impls are adjacent.