[llc] Default MCUseDwarfDirectory to true

For Clang, `MCUseDwarfDirectory` is true by default for the majority cases
(-fintegrated-as or -gdwarf-5; most targets use -fintegrated-as by default).
Defaulting MCUseDwarfDirectory to true can reduce the differences between clang
and llc.

Reviewed By: #debug-info, dblaikie

Differential Revision: https://reviews.llvm.org/D105856