[clang][NewPM] Do not eliminate available_externally durng `-O2 -flto` runs

This fixes CodeGen/available-externally-suppress.c when the new pass manager is
turned on by default. available_externally was not emitted during -O2 -flto
runs when it should still be retained for link time inlining purposes. This can
be fixed by checking that we aren't LTOPrelinking when adding the
EliminateAvailableExternallyPass.

Differential Revision: https://reviews.llvm.org/D63580

llvm-svn: 363971