Add a thread-local ring buffer of heap allocations

Summary:
We need this in order to properly report heap-use-after-free,
since we don't have a quarantine.

This is a first part of the code, more like a proof of concept.
But I'd like to commit at as is and proceed with refactoring,
adding a ThreadRegistry, and extending the functionality.

Reviewers: eugenis

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D51394

llvm-svn: 340971