swift234

[Serialization] Sanity-check alignment of AST data.

Somewhere--and I forget where--either the LLVM bitcode system or the
OnDiskHashTable implementation depends on 32-bit alignment being preserved.
We had thought that only alignment /from the start of the stream/ mattered,
but it looks like the whole data section has to be 32-bit-aligned.

<rdar://problem/17814086>

Swift SVN r20599