Work around MSVC's lack of support for unrestricted unions by making this

struct a bit bigger under MSVC (this shouldn't be a big deal; we typically
allocate no more than two of these at a time, on the stack).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276509 91177308-0d34-0410-b5e6-96231b3b80d8