Add parallel loop metadata to cilk_for loops

- add CGLoopInfoStack to track loop nesting information
- modify cilk_for code emission to push and pop the LoopInfoStack
- change CGBuilder to use a custom inserter which add parallel loop metadata
  when building inside of a parallel loop.
Mark each call to llvm.sjlj.setjmp as returns_twice
- Also mark __cilk_sync as returns_twice so it will be inlined
- Fixes incorrect behavior in some cases (eg, spawning functions
  containing floating-point arithmetic) at O3