WholeProgramDevirt: Move logic for finding devirtualizable call sites to Analysis.

The plan is to eventually make this logic simpler, however I expect it to
be a little tricky for the foreseeable future (at least until we're rid of
pointee types), so move it here so that it can be reused to build a summary
index for devirtualization.

Differential Revision: http://reviews.llvm.org/D20005

llvm-svn: 269081