[ELF] - Fix for "Bug 34238 - LTO is optimizing away symbols referenced from linker scripts"

Code suggested by Rui Ueyama in PR34238 comments.

Previously LTO optimized away symbols referenced from linker script
because did not see that them are used from regular objects.

Patch adds such symbols as undefined earlier, before running LTO,
what sets IsUsedInRegularObj for them and fixes the issue.

Differential revision: https://reviews.llvm.org/D37009

llvm-svn: 311534