[Support] Renamed member 'Size' to 'AllocatedSize' in MemoryBlock and OwningMemoryBlock.

Rename member 'Size' to 'AllocatedSize' in order to provide a hint that the
allocated size may be different than the requested size. Comments are added to
clarify this point.  Updated the InMemoryBuffer in FileOutputBuffer.cpp to track
the requested buffer size.

Patch by Machiel van Hooren. Thanks Machiel!

https://reviews.llvm.org/D61599

llvm-svn: 361195