[llvm-cxxfilt]Add test to show that empty lines can be handled

I recently discovered a bug in llvm-cxxfilt introduced in r353743 but
was fixed later incidentally due to r355031. 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.

Reviewed by: mattd

Differential Revision: https://reviews.llvm.org/D58785

llvm-svn: 355929