MemoryBlock: Do not automatically extend a given size to a multiple of page size.

Previously, MemoryBlock automatically extends a requested buffer size to a
multiple of page size because (I believe) doing it was thought to be harmless
and with that you could get more memory (on average 2KiB on 4KiB-page systems)
"for free".

That programming interface turned out to be error-prone. If you request N
bytes, you usually expect that a resulting object returns N for `size()`.
That's not the case for MemoryBlock.

Looks like there is only one place where we take the advantage of
allocating more memory than the requested size. So, with this patch, I
simply removed the automatic size expansion feature from MemoryBlock
and do it on the caller side when needed. MemoryBlock now always
returns a buffer whose size is equal to the requested size.

Differential Revision: https://reviews.llvm.org/D56941

llvm-svn: 351916