Driver: Support invoking Clang on .ll or .bc inputs.

 - We actually pretend that we have two separate types for LLVM assembly/bitcode because we need to use the standard suffixes with LTO ('clang -O4 -c t.c' should generate 't.o').

It is now possible to do something like:
  $ clang -emit-llvm -S t.c -o t.ll ... assorted other compile flags ...
  $ clang -c t.ll -o t.o ... assorted other compile flags ...
and expect that the output will be almost* identical to:
  $ clang -c t.c -o t.o ... assorted other compile flags ...
because all the target settings (default CPU, target features, etc.) will all be initialized properly by the driver/frontend.

*: This isn't perfect yet, because in practice we will end up running the optimization passes twice. It's possible to get something equivalent out with a well placed -mllvm -disable-llvm-optzns, but I'm still thinking about the cleanest way to solve this problem more generally.

llvm-svn: 105584