Certain functions in glibc have "nonnull" attributes on pointer
parameters (even in cases where passing a null pointer should be handled
correctly). There are a few cases of such calls in flang: memcmp and
memcpy with the length parameter set to 0.
Avoid passing a null pointer to these functions, since the conflict with
the nonnull attribute could cause an undefined behavior.
This was detected by the undefined behavior sanitizer.
These functions will be used as part of the HLFIR lowering for
forall/where. The contents of the API were requested by @jeanPerier.
The API is designed around that use case, assuming that the caller knows
through some side channel what size to allocate for boxes returned from
the pop() function.
Differential Revision: https://reviews.llvm.org/D150050