Add allocator_frees_and_returns_null_on_realloc_zero=false flag for compatibility...

Add allocator_frees_and_returns_null_on_realloc_zero=false flag for compatibility with allocators which allow a realloc(p, 0) and don't free the pointer.

Summary:
I know of two implementations that do this (ASan is not protecting against accessing the returned memory for now, just like malloc(0)):
SIE libc on the PS4
dlmalloc has a flag for this

This allows us to properly support this behaviour.

Reviewers: vsk, kcc

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D31295

llvm-svn: 299016