[llvm-dwarfdump][Statistics] Make calculations of vars in global scope more accurate

It isn't known how many times we've seen the same variable or member in
the global scope (unlike in functions), but there still can be some duplicates
among different CUs.
So, this patch proposes to count variables in the global scope just as a sum of
the number of vars, constant members and artificial entities.

Reviewed by: aprantl

Differential Revision: https://reviews.llvm.org/D73004