WholeProgramDevirt: Change internal vcall data structures to match summary.

Group calls into constant and non-constant arguments up front, and use uint64_t
instead of ConstantInt to represent constant arguments. The goal is to allow
the information from the summary to fit naturally into this data structure in
a future change (specifically, it will be added to CallSiteInfo).

This has two side effects:
- We disallow VCP for constant integer arguments of width >64 bits.
- We remove the restriction that the bitwidth of a vcall's argument and return
  types must match those of the vfunc definitions.
I don't expect either of these to matter in practice. The first case is
uncommon, and the second one will lead to UB (so we can do anything we like).

Differential Revision: https://reviews.llvm.org/D29744

llvm-svn: 295110