[Index] Add setPreprocessor member to IndexDataConsumer.

Description

[Index] Add setPreprocessor member to IndexDataConsumer.

Summary:
This enables us to use information in Preprocessor when handling symbol
occurrences.

Reviewers: arphaman, hokein

Reviewed By: hokein

Subscribers: malaperle, cfe-commits

Differential Revision: https://reviews.llvm.org/D40884

Details