[analyzer] Malloc: unique leak reports by allocation site.

When we find two leak reports with the same allocation site, report only
one of them.

Provide a helper method to BugReporter to facilitate this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151287 91177308-0d34-0410-b5e6-96231b3b80d8