Don't generate assembly in backend diagnostic tests

A leftover -S was generating unwanted output in the source tree overriding
-only flags that normally disable output.

This reverts commit r210323 and implements the proper fix.

Reported by Timur Iskhodzhanov!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210326 91177308-0d34-0410-b5e6-96231b3b80d8