Fix buggy Twine storage in ELFLinkingContext::searchLibrary()

This patch fixes a forbidden use of Twine. It should only be used
as an intermediary value, but never stored.

This caused a bug in lld when running on Linux and compiled with
optimizations - it couldn't properly search libs.

Patch from Rafael Auler!

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218083 91177308-0d34-0410-b5e6-96231b3b80d8