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

[jhenderson]

Summary

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.

[mattd]

LGTM

test/tools/llvm-cxxfilt/empty-line.test

+# Show that llvm-cxxfilt can handle empty lines in the input.
+RUN: echo "" > %t
+RUN: echo "_Z3foov" >> %t
+RUN: echo "" >> %t
+RUN: llvm-cxxfilt < %t | FileCheck %s
+
+CHECK:       {{^$}}
+CHECK-NEXT:  foo()
+CHECK-EMPTY: