LTO: Assert visibility of local linkage when merging symbols

`ModuleLinker::getLinkageResult()` shouldn't create symbols with local
linkage and non-default visibility -- in fact, symbols with local
linkage shouldn't be merged at all.  Assert to that effect.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208262 91177308-0d34-0410-b5e6-96231b3b80d8