Improve zero-size allocation with safe_malloc, etc.

Summary:
The current implementations of the memory allocation functions mistake a nullptr returned from std::malloc, std::calloc, or std::realloc as a failure. The behaviour for each of std::malloc, std::calloc, and std::realloc when the size is 0 is implementation defined (ISO/IEC 9899:2018 7.22.3), and may return a nullptr.

This patch checks if space requested is zero when a nullptr is returned, retry requesting non-zero if it is.

Authored By: andusy

Reviewers: hubert.reinterpretcast, xingxue, jasonliu

Reviewed By: hubert.reinterpretcast, xingxue, abrachet

Subscribers: abrachet, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63668

llvm-svn: 364322