Improve driver error message when only running the preprocessor and an input is

already preprocessed.

--
ddunbar@giles:tmp$ touch t.i
ddunbar@giles:tmp$ gcc -E t.i
ddunbar@giles:tmp$ clang -E t.i
clang: warning: t.i: previously preprocessed input unused when '-E' is present
ddunbar@giles:tmp$ 
--

<rdar://problem/6813375> [driver] driver prints confusing message when running -E on preprocessed file

llvm-svn: 82120