Remove ignore-empty-index-file option

Summary:
As discussed in the D32195 review thread and on IRC, remove this option
and replace with parameter, which will be set to true when invoked
from clang in the context of a ThinLTO distributed backend.

Reviewers: pcc

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D33133

llvm-svn: 302939