Remove ignore-empty-index-file option

Summary:
Clang changes to remove this option and replace with a parameter
always set in the context of a ThinLTO distributed backend.

Depends on D33133.

Reviewers: pcc

Subscribers: mehdi_amini, eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D33134

llvm-svn: 302940