[gold/ThinLTO] Restore ThinLTO file management in gold plugin

Summary:
The gold-plugin changes added along with the new LTO API in r278338 had
the effect of removing the management of the PluginInputFile that
ensured the files weren't released back to gold until the backend
threads were complete. Add back the old file handling.

Fixes PR29020.

Reviewers: mehdi_amini

Subscribers: mehdi_amini, llvm-commits, hjl.tools

Differential Revision: https://reviews.llvm.org/D23721

llvm-svn: 279356