DebugInfo: Don't include size/alignment on class declarations

This seems like it must've been a leftover by accident - no tests were
backing it up & it doesn't make much sense to include size/alignment on
class declarations (it'd only be on those declarations for which the
definition was available - otherwise the size/alignment would not be
known).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290631 91177308-0d34-0410-b5e6-96231b3b80d8