PMBuilder: Expose an option to disable tail calls

Adds API to allow frontends to disable tail calls in PassManagerBuilder.

<rdar://problem/16050591>
BackendUtil: Pass through -mdisable-tail-calls

The frontend option -fno-optimize-sibling-calls resolves to -cc1's
-mdisable-tail-calls, which is passed to the TargetMachine in the
backend.  PassManagerBuilder was adding the -tailcallelim pass anyway.

Use a new DisableTailCalls option in PassManagerBuilder to disable tail
calls harder.

Requires the matching commit in LLVM that adds DisableTailCalls.

<rdar://problem/16050591>