Improve BOM test from r195877

 1) Use %clang_cc1 instead of the driver
 2) Validate that the input contains a BOM
 3) Validate that the BOM has been stripped from the output

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195886 91177308-0d34-0410-b5e6-96231b3b80d8