ThinLTO: Remove check for multiple modules before applying weak resolutions.

ThinLTO: Remove check for multiple modules before applying weak resolutions.

This check is not only unnecessary, it can produce the wrong result. If we
are linking a single module and it has an exported linkonce symbol, we need
to promote to weak in order to avoid PR19901-style problems.

Differential Revision: http://reviews.llvm.org/D21917

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274722 91177308-0d34-0410-b5e6-96231b3b80d8