Reorder members to save padding.

There's more potential here, but these Exprs aren't used that often so I don't feel like doing heroic bit packing right now.

-8 bytes on every class changed (x86_64).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151501 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed