[ThinLTO] Fix missing call graph edges for calls with bitcasts.

This change doesn't fix the root cause of the miscompile PR34966 as the root
cause is in the linker ld64. This change makes call graph more complete
allowing to have better module imports/exports.

rdar://problem/35344706

Reviewers: tejohnson

Reviewed By: tejohnson

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

Differential Revision: https://reviews.llvm.org/D39356

llvm-svn: 317853