[ThinLTO] Handle chains of aliases

At -O0, globalopt is not run during the compile step, and we can have a
chain of an alias having an immediate aliasee of another alias. The
summaries are constructed assuming aliases in a canonical form
(flattened chains), and as a result only the base object but no
intermediate aliases were preserved.

Fix by adding a pass that canonicalize aliases, which ensures each
alias is a direct alias of the base object.

Reviewers: pcc, davidxl

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

Differential Revision: https://reviews.llvm.org/D54507

llvm-svn: 350423