Driver: Handle -flto, -O4, and tweak -emit-llvm to match llvm-gcc.

 - -emit-llvm no longer changes what compilation steps are done.

 - -emit-llvm and -emit-llvm -S write output files with .o and .s
    suffixes, respectively.

 - <rdar://problem/6714125> clang-driver should support -O4 and -flto,
   like llvm-gcc

llvm-svn: 67645