Reapply r198845, reverted in r198849, with a fix to make it valid C++98, not
just valid C++11 =)

Original commit message:

PR18427: Use an appropriately-aligned buffer in APValue, to avoid a crash on
SPARC, where uint64_t apparently requires higher alignment than void*.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198903 91177308-0d34-0410-b5e6-96231b3b80d8