[llvm-cov] Attempt to fix a test failure on Windows

Don't make the test/tools/llvm-cov/demangle.test depend on the order in
which symbols are seen, or on the exact formatting llvm-cov emits after
a symbol is printed. This is an attempt to fix a Windows bot failure:

  http://lab.llvm.org:8011/builders/clang-x86-win2008-selfhost/builds/9141

I don't know what the root cause of the failure is, or why the
showTemplateInstantiations test doesn't fail in the same way on the
Windows bots. However, this measure can't hurt, and it'll at least get
me on the blamelists again.

llvm-svn: 275758