Limit CFI blacklist entries to specific CFI mode

Summary:
Since D37924 and D37925 were merged, it's now possible to specify
individual sanitizers or CFI modes in sanitizer blacklists. Update the
CFI blacklist entries to only apply to cfi-unrelated-cast checks.

Reviewers: eugenis, pcc

Reviewed By: eugenis

Subscribers: kcc

Differential Revision: https://reviews.llvm.org/D38385

llvm-svn: 315216