[ThinLTO] Scan all variants of vague symbol for reachability.

Summary:
Alias can make one (but not all) live, we still need to scan all others if this symbol is reachable
from somewhere else.

Reviewers: tejohnson, grimar

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D56117

llvm-svn: 350269