[sanitizer] Implement reallocarray.

Summary:
It's a cross of calloc and realloc. Sanitizers implement calloc-like check for size
overflow.

Reviewers: vitalybuka, kcc

Subscribers: kubamracek, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61108

llvm-svn: 359708