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

One step toward differentiating following two commands:
   clang -O3 -flto a.c -c, and 
   clang -O3 -emit-llvm a.c

Thanks many awesome folks for clarifying things. 


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189148 91177308-0d34-0410-b5e6-96231b3b80d8