Update langref to indicate that calls may be convergent.

Summary:
As previously written, only functions could be convergent.  But calls
need to have a notion of convergence as well.

To see why this is important, consider an indirect call.  We may or may
not want to disable optimizations around it and behave as though we're
calling a convergent function -- it depends on the semantics of the
language we're compiling.  Thus the need for this attr on the call.

Reviewers: jingyue, joker.eph

Subscribers: llvm-commits, tra, jhen, arsenm, chandlerc, hfinkel, resistor

Differential Revision: http://reviews.llvm.org/D17314

llvm-svn: 261111