ELF: Re-implement -u directly and remove CanKeepUndefined flag.

The semantics of the -u flag are to load the lazy symbol named by the flag. We
were previously relying on this behavior falling out of symbol resolution
against a synthetic undefined symbol, but that didn't quite give us the
correct behavior, so we needed a flag to mark symbols created with -u so
we could treat them specially in the writer. However, it's simpler and less
error prone to implement the required behavior directly and remove the flag.

This fixes an issue where symbols loaded with -u would receive hidden
visibility even when the definition in an object file had wider visibility.

Differential Revision: http://reviews.llvm.org/D19560

llvm-svn: 267639