[llvm-dwarfdump][Statistics] Distinguish parameters with same name or w/o a name

A few DW_TAG_formal_parameter's of the same function may have the same
name (e.g. variadic (template) functions) or don't have a name at all
(if the parameter isn't used inside the function body), but we still
need to be able to distinguish between them to get correct number of 'total vars'
and 'availability' metric.

Reviewed by: aprantl

Differential Revision: https://reviews.llvm.org/D73003