[asan] Fix comparison in BufferedStackTrace::LocatePcInTrace

Summary:
Debug builds can have larger distance between stack trace and PC on that stack.
If we assume that PC is always correct we can snap it to the nearest trace.

Reviewers: eugenis

Subscribers: llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D28342

llvm-svn: 291173