[NFC][DwarfDebug] Prefer explicit to auto type deduction

We should use explicit type instead of auto type deduction when
the type is so obvious. In addition, we remove ambiguity, since auto
type deduction sometimes is not that intuitive, so that could lead
us to some unwanted behavior.

This patch fixes that in the collectCallSiteParameters() from
DwarfDebug module.

Differential Revision: https://reviews.llvm.org/D79624