[libc] Improve testing of mem functions

This patch extracts the testing logic from `op_tests.cpp` into
`memory_check_utils.h` so we can reuse it for mem* function integration
tests.

This makes testing consistent and thorough.
For instance this catches a bug that got unnoticed during submission of
D136595 and D135134. Integration test for memcmp was only testing a
single size.

This also leverages ASAN to make sure that data is not read / written
outside permitted boundaries

Differential Revision: https://reviews.llvm.org/D136865