Merge StreamableMemoryObject into MemoryObject.

Every MemoryObject is a StreamableMemoryObject since the removal of
StringRefMemoryObject, so just merge the two.

I will clean up the MemoryObject interface in the upcoming commits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221766 91177308-0d34-0410-b5e6-96231b3b80d8