[LTO] Don't internalize declarations

Summary:
Some links were failing with "Global is external, but doesn't have
external or weak linkage!" in ThinLTO builds with debug
information. This happened when we elide the body of a global that is
referenced by debug info. This results in a declaration, which we
would then internalize - but declarations cannot be internal. This
change avoids the problem by not internalizing these declarations.

Fixes PR38046.

Reviewers: pcc, tejohnson

Subscribers: mehdi_amini, aprantl, hiraditya, JDevlieghere, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49777

llvm-svn: 338100