MemoryBuffer: Increase the alignment of small file buffers to 16

This was manifesting as an LLVM_ASSUME_ALIGNED() failure in an ELF debug
info test when building LLVM with clang in the Microsoft C++ ABI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197401 91177308-0d34-0410-b5e6-96231b3b80d8