[llvm-dwarfdump] Fix split-dwarf bug in stats for inlined var loc cov

Initial (D96045) patch didn't handle split dwarf cases,
so this fixes that bug.

In addition, before applying this patch, we had a slowdown
that happened after the D96045. With this patch,
the slowdown will be fixed as well.

Differential Revision: https://reviews.llvm.org/D100951