[llvm] Skip over empty line table entries.

Summary:
This is similar to how addr2line handles consecutive entries with the
same address - pick the last one.

Reviewers: dblaikie, friss, JDevlieghere

Reviewed By: dblaikie

Subscribers: ormris, echristo, JDevlieghere, probinson, aprantl, hiraditya, rupprecht, jdoerfert, llvm-commits

Tags: #llvm, #debug-info

Differential Revision: https://reviews.llvm.org/D58952

llvm-svn: 355972