This commit makes use of the newly created MultiMemRead packet to
provide an efficient implementation of MultiMemRead inside
ProcessGDBRemote.
Testing is tricky, but it is accomplished two ways:
1. Some Objective-C tests would fail if this were implemented incorrectly,
as there is already an in-tree use of the base class implementation of
MultiMemRead, which is now getting replaced by the derived class.
2. One Objective-C test is modified so that we ensure the packet is
being sent by looking at the packet logs. While not the most elegant
solution, it is a strategy adopted in other tests as well. This gets
around the fact that we cannot instantiate / unittest a mock
ProcessGDBRemote.
Depends on https://github.com/llvm/llvm-project/pull/163651
276bccda66