I recently discovered a bug in llvm-cxxfilt introduced in rL353743 but was fixed later incidentally due to rL355031. Specifically, llvm-cxxfilt was attempting to call .back() on an empty string any time there was a new line in the input. This was causing a crash in my debug builds only. This patch simply adds a test that explicitly tests that llvm-cxxfilt handles empty lines correctly. It may pass under release builds under the broken behaviour, but it fails at least in debug builds.
Note: a FileCheck assertion means that D58784 needs to land before this test can land.