uselistorder: -mllvm -preserve-bc-use-list-order => -emit-llvm-uselists

Stop relying on `cl::opt` to pass along the driver's decision to
preserve use-lists.  Create a new `-cc1` option called
`-emit-llvm-uselists` that does the right thing (when -emit-llvm-bc).
Note that despite its generic name, it *doesn't* do the right thing when
-emit-llvm (LLVM assembly) yet.  I'll hook that up soon.

This doesn't really change the behaviour of the driver.  The default is
still to preserve use-lists for `clang -emit-llvm` and `clang
-save-temps`, and nothing else.  But it stops relying on global state
(and also is a nicer interface for hackers using `clang -cc1`).

llvm-svn: 234962