[ThinLTO] Add testing of summary index parsing to a couple CFI tests

Summary:
Changes to some clang side tests to go with the summary parsing patch.

Depends on D47905.

Reviewers: pcc, dexonsmith, mehdi_amini

Subscribers: inglorion, eraman, cfe-commits, steven_wu

Differential Revision: https://reviews.llvm.org/D47906

llvm-svn: 335618