Per feedback from Eli, recognize in the transfer function logic for

Per feedback from Eli, recognize in the transfer function logic for
__builtin_offsetof in the static analyzer that __builtin_offsetof is
not guaranteed to return an integer constant.  We will need to shore
this up later, but now at least we have correct support for when this
*is* an integer constant.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@81830 91177308-0d34-0410-b5e6-96231b3b80d8