Driver::IsUsingLTO() no longer return true when seeing -emit-llvm.

The rationale for this change is to differentiate following two situations:
  1) clang -c -emit-llvm a.c
  2) clang -c -flto a.c 

Reviewed by Eric Christopher. Thanks a lot!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188352 91177308-0d34-0410-b5e6-96231b3b80d8