Whenever reasonable, merge ASAN quarantine batches to save memory.

Summary:
There are cases when thread local quarantine drains almost empty
quarantine batches into the global quarantine. The current approach leaves
them almost empty, which might create a huge memory overhead (each batch
is 4K/8K, depends on bitness).

Reviewers: eugenis

Subscribers: kubabrecka, llvm-commits

Differential Revision: https://reviews.llvm.org/D28068

llvm-svn: 292525