[DebugInfo] Enable debug info emission for extern variables in C++

Debug info emission for extern variables in C++ was previously disabled
when the functionality was added in https://reviews.llvm.org/D71818 and
originally in https://reviews.llvm.org/D70696, because there was no use
case. We are enabling it now, as we start to deploy BPF programs
compiled from C++, leveraging C++ features like templates to reduce code
complexity. This patch is required so that we can still use kconfig in
such BPF programs compiled from C++.

Reviewed By: rnk, dblaikie, MaskRay, yonghong-song

Differential Revision: https://reviews.llvm.org/D153898