[llvm-dwp] Abort when dwo_id is unset

Summary:
An empty dwo_id indicates a degenerate .dwo file that should not have been generated in the first place. Instead of discovering this error later when merging with another degenerate .dwo file, print an error immediately when noticing an unset dwo_id, including the filename of the offending file.

Test case created by compiling a trivial file w/ `-fno-split-dwarf-inlining -gmlt -gsplit-dwarf -c` prior to r353771

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: jdoerfert, aprantl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58085

llvm-svn: 353846