Return memory to OS right after free (not in the async thread).

Summary:
In order to avoid starting a separate thread to return unused memory to
the system (the thread interferes with process startup on Android,
Zygota waits for all threads to exit before fork, but this thread never
exits), try to return it right after free.

Reviewers: eugenis

Subscribers: cryptoad, filcab, danalbert, kubabrecka, llvm-commits

Patch by Aleksey Shlyapnikov.

Differential Revision: https://reviews.llvm.org/D27003

llvm-svn: 288091