[ThinLTO] Remove too aggressive assertion in building function call graph.

The assertion was introduced in r317853 but there are cases when a call
isn't handled either as direct or indirect. In this case we add a
reference graph edge but not a call graph edge.

Reviewers: tejohnson

Reviewed By: tejohnson

Subscribers: mehdi_amini, inglorion, eraman, hiraditya, efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D40056

llvm-svn: 318540