Apply ABSL_MUST_USE_RESULT annotation to MemoryModifyFunction.
Apply ABSL_MUST_USE_RESULT annotation to MemoryModifyFunction.

All exising calls now consume the return value.

PiperOrigin-RevId: 485713993
Change-Id: I178a751da42dc38ac2fae2c250409e7185cfc99f

google/tcmalloc,google/tcmalloc,google/tcmalloc