[Sanitizers] Rename CallocShouldReturnNullDueToOverflow to CheckForCallocOverflow

Summary:
Due to changes in semantics, CheckForCallocOverflow makes much more sense
now.

Reviewers: eugenis

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D34799

llvm-svn: 306747