[ThinLTO] Release files read when creating combined index in gold plugin

This wasn't causing an issue since at HEAD we exit the linker completely
after creating the combined index.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255156 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed