Minor performance improvement to not do unnecessary work
in my last patch. // rdar://14989999


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@193441 91177308-0d34-0410-b5e6-96231b3b80d8
Skip this test when using lit integrated shell

It wasn't passing on UNIX with LIT_USE_INTERNAL_SHELL=1
due to cat globbing.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@193443 91177308-0d34-0410-b5e6-96231b3b80d8