[NFC][DwarfDebug] Avoid default capturing when using lambdas

It is bad practice to capture by default (via [&] in this case) when
using lambdas, so we should avoid that as much as possible.

This patch fixes that in the getForwardingRegsDefinedByMI
from DwarfDebug module.

Differential Revision: https://reviews.llvm.org/D79616