[LLDB] Fix AddressSanitizer failure in MemoryCache

The lldb sanitizer bot is flagging a container-overflow error after we
introduced test TestWasm.py. MemoryCache::Read didn't behave correctly
in case of partial reads that can happen with object files whose size is
smaller that the cache size. It should return the actual number of bytes
read and not try to fill the buffer with random memory.
Module::GetMemoryObjectFile needs to be modified accordingly, to resize
its buffer to only the size that was read.

Differential Revision: https://reviews.llvm.org/D75200