[SystemZ][z/OS] Add AutoConvert.h header to MemoryBuffer.cpp

This commit https://github.com/llvm/llvm-project/commit/75e164f61d391979b4829bf2746a5d74b94e95f2 removed the AutoConvert.h header causing a build break on z/OS. This patch adds it back to fix it.

Reviewed By: zibi

Differential Revision: https://reviews.llvm.org/D118129