[lldb] Make MemoryCache::Read more resilient

MemoryCache::Read is not resilient to partial reads when reading memory
chunks less than or equal in size to L2 cache lines. There have been
attempts in the past to fix this but nothing really solved the root of
the issue.

I first created a test exercising MemoryCache's implementation and
documenting how I believe MemoryCache::Read should behave. I then
rewrote the implementation of MemoryCache::Read as needed to make sure
that the different scenarios behaved correctly.

rdar://105407095

Differential Revision: https://reviews.llvm.org/D145624