[DebugInfo] Remove call sites when eliminating unreachable blocks

Summary:
When eliminating an unreachable block we must remove any call site
information for calls residing in the block.

This was originally found on a downstream target, and the attached x86
test case was produced by hand-modifying some MIR.

Reviewers: aprantl, asowda, NikolaPrica, djtodoro, ivanbaev, vsk

Reviewed By: NikolaPrica, vsk

Subscribers: vsk, hiraditya, llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D64500

llvm-svn: 368566