Clean up use of C allocation functions

If the value returned by `malloc`, `calloc` or `realloc` is not checked
for null pointer, this change replaces them for `safe_malloc`,
`safe_calloc` or `safe_realloc`, which are defined in the namespace `llvm`.
These function report fatal error on out of memory.

In the plain C files, assertion statements are added to ensure that memory
is successfully allocated.

The aim of this change is to get better diagnostics of OOM on Windows.

Differential Revision: https://reviews.llvm.org/D43017

llvm-svn: 325661