Improve the debug-info test created in r274263.
This patch is related to r274263 or Phabricator/D21818.
This patch aims to improve the test case added in the previous commit to verify
specifically that the stack protector pass is adding the debug line info as
intended. Before, the test only verified that the verifier pass does not crash.
The current approach is to generate the assembly output and then look for the
Differential Revision: https://reviews.llvm.org/D25290