[ObjectFileMachO] Disable memory caching for savecore.

Summary:
It's not really useful, and largely increases the footprint.

<rdar://problem/49293525>

Reviewers: jasonmolenda

Subscribers: llvm-commits, lldb-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59896

llvm-svn: 357134