[LCG] Add aliased functions as LCG roots

Current LCG doesn't check aliased functions. So if an internal function has a public alias it will not be added to CG SCC, but it is still reachable from outside through the alias.
So this patch adds aliased functions to SCC.

Differential Revision: https://reviews.llvm.org/D59898

llvm-svn: 357795