LTO: Do summary-based prevailing symbol resolution at --lto-O0.

Prevailing symbol resolution is necessary for correctness. Without
this we can end up dropping a referenced linkonce symbol from the link.

Differential Revision: https://reviews.llvm.org/D33570

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303939 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed