Add backend passes (inlining etc) to cling.

They are usually provided by clang's BackendUtil - which we don't use and which
has no support for incremental ("streaming") compilation but runs the passes on
the whole module the whole time, and does end-of-TU cleanup (e.g. "dead" code
removal) that we cannot use because subsequent transactions might create new
uses - think force_inline functions that must stay.

Instead BackendPass wraps what's in clang's BackendUtil into a
TransactionTransformer. It gets added to the IR transformers. In the future this
should only transform the delta of one Transaction instead of the whole Module.

This fixes the libc++ issue we worked around by
templates' symbols are not exported anymore and cause missing symbols with
XCode 5.1.

We include ObjC passes even though this blows up the list of linked libraries to
make it easier to port cling to ObjC(++).
Disable module validation: it takes ages.
Disable module verification explicitly.
Register the inliner pass: it is a module pass.
Implement custom inliner to skip doFinalization.