[Driver] Fix the case when use -fembed-bitcode and -flto= together

Summary:
-fembed-bitcode was only checking for old style LTO flag (-flto) but not
considering the new -flto= style option. That makes clang output bitcode
embedded in bitcode object when using -flto= and -fembed-bitcode= together.
Now clang should output normal bitcode file when using LTO and ignores
-fembed-bitcode option.

Reviewers: joker.eph

Subscribers: joker.eph, cfe-commits

Differential Revision: http://reviews.llvm.org/D20374

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269961 91177308-0d34-0410-b5e6-96231b3b80d8