[ThinLTO] Convert dead alias to declarations

Summary:
This complements the fixes in r323633 and r324075 which drop the
definitions of dead functions and variables, respectively.

Fixes PR36208.

Reviewers: grimar, rafael

Subscribers: mehdi_amini, llvm-commits, inglorion

Differential Revision: https://reviews.llvm.org/D42856

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324242 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed