Fix strip-dead-debug-info test if path contains "bar".

This test checks that the string 'bar' (no quotes) doesn't exist in the
output after running opt.  But opt embeds the absolute path to the
filename, and on my machine, the filename contains the string 'jlebar',
causing the test to fail.

This patch changes the test to look for the string '"bar"' instead.

llvm-svn: 272941