[LinkerWrapper] Fix use of string savers and correctly pass bitcode libraries

This patch removes some uses of string savers that are no-longer needed.
We also create a new string saver when linking bitcode files. It seems
that occasionally the symbol string references can go out of scope when
they are added to the LTO input so we need to save these names that are
used for symbol resolution. Additionally, a previous patch added new
logic for handling bitcode libraries, but failed to actually add them to
the input. This bug has been fixed.

Fixes #56445

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D129383