[ThinLTO] Keep available_externally symbols live

Summary:
This change fixes PR36483. The bug was originally introduced by a change
that marked non-prevailing symbols dead. This broke LowerTypeTests
handling of available_externally functions, which are non-prevailing.
LowerTypeTests uses liveness information to avoid emitting thunks for
unused functions.

Marking available_externally functions dead is incorrect, the functions
are used though the function definitions are not. This change keeps them
live, and lets the EliminateAvailableExternally/GlobalDCE passes remove
them later instead.

I've also enabled EliminateAvailableExternally for all optimization
levels, I believe it being disabled for O1 was an oversight.

Reviewers: pcc, tejohnson

Reviewed By: tejohnson

Subscribers: grimar, mehdi_amini, inglorion, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D43690

llvm-svn: 327041